Aquarium Volume Calculator: US Gallons from Tank Dimensions
Enter your tank’s length, width, and height in inches to instantly calculate exact US gallons. Supports rectangular, bowfront, and hexagonal aquariums. Displays gross volume, net usable gallons, liters, freshwater and saltwater weight, and a basic stocking guide. Free — no signup required.
Understanding Aquarium Volume in US Gallons: The Foundation of Fishkeeping
Every decision you make as an aquarist flows from one number: your tank’s water volume. The gallons figure determines what filter you buy, how much dechlorinator you dose, how many fish your tank can support, how much salt mix goes into a reef tank, what heater wattage you need, and whether your floor can hold the weight. Getting this number right at the start saves you from dozens of costly mistakes down the road.
The official US gallon is defined by federal law as exactly 231 cubic inches. That’s not an approximation or a rounding convention — it’s a legal standard. One US gallon equals 231 cubic inches, period. This means that for any rectangular aquarium, the math is completely deterministic: multiply the inside length by the inside width by the inside height in inches, then divide by 231. The result is your gross volume in US gallons. No estimation, no guessing, no tables to look up.
Where things get complicated is that commercial aquarium labels are nominal designations, not precise measurements. A tank marketed as “55 gallons” at your local fish store typically has inside dimensions of approximately 47.5 x 12.75 x 20.75 inches, which calculates to 54.9 gallons — very close to 55, but not exactly 55. The same is true across all standard sizes. A “75-gallon” tank usually calculates to 74.5 to 75.5 gallons depending on the manufacturer. For casual fishkeeping, this small discrepancy never matters. But when you’re mixing saltwater to a specific salinity, medicating a sick fish at a precise dosage, or selecting equipment rated to exact gallon ranges, knowing your actual volume gives you a meaningful edge in accuracy.
Why Water Volume Controls Everything in Your Tank
New hobbyists often underestimate how fundamentally water volume governs a tank’s stability. The relationship is almost paradoxical: a larger aquarium is generally easier to maintain than a smaller one, even though it holds more water and requires more equipment. The reason is dilution. In a 10-gallon tank, one dead fish or one uneaten food pellet can spike ammonia to dangerous levels within hours. In a 55-gallon tank, the same event barely registers. Water chemistry problems that would crash a 5-gallon in a day can take a week to develop in a 75-gallon, giving you time to detect and correct them before fish are in danger.
This is why aquarists who start with small tanks and switch to larger ones often say the hobby became more enjoyable overnight. They stopped fighting water chemistry disasters and started actually watching their fish. If you’re a beginner, the “start small” advice you’ll hear at pet stores is, frankly, bad advice in most cases. A 29-gallon or 40-gallon breeder tank is a far better starting point than a 10-gallon.
Freshwater vs Saltwater: Why the Density Difference Matters
Freshwater at typical aquarium temperatures (between 68 and 78 degrees Fahrenheit) weighs 8.34 lbs per US gallon. Saltwater at typical marine aquarium salinity — a specific gravity of 1.025, which corresponds to approximately 35 parts per thousand salt — weighs approximately 8.55 lbs per US gallon. That’s a difference of about 2.5 percent per gallon.
On small tanks, that difference is negligible. On large tanks, it adds up quickly. A 150-gallon saltwater system holds water that weighs 150 x 8.55 = 1,282.5 lbs compared to 150 x 8.34 = 1,251 lbs for freshwater — a difference of 31.5 lbs. That’s the weight of a large bag of dog food sitting on top of your tank stand that wasn’t there in your freshwater setup. When you’re calculating floor loads for large marine tanks, always use the saltwater weight figure.
Additionally, saltwater is slightly more buoyant, which affects how decorations, live rock, and certain protein skimmer components behave. And because saltwater evaporates its water component but leaves the salt behind, the specific gravity rises as the tank evaporates. A reef keeper with a 100-gallon display tank and a 30-gallon sump who doesn’t top off for two days can see salinity drift measurably — knowing the exact combined water volume of both vessels helps calculate the precise freshwater top-off needed to restore target salinity.
Net Volume vs Gross Volume: The Number That Actually Matters
Gross volume is the mathematical volume of the empty tank based on its inside dimensions. Net usable volume is the actual water volume after accounting for everything that occupies space inside: substrate (gravel, sand, aragonite), live rock, decorations, equipment like internal filters and heaters, and the air gap at the top where you don’t fill the tank all the way to the rim.
For a lightly decorated freshwater tank with a 1-inch gravel bed and minimal decor, net volume runs about 90 to 95 percent of gross. For a heavily aquascaped aquarium with dense hardscape, root driftwood, and thick substrate, net volume can drop to 80 percent of gross. For a reef tank with significant live rock (the traditional 1 to 1.5 lbs per gallon recommendation), net volume after accounting for all the rock can be 75 to 85 percent of gross.
This calculator uses 90 percent as the net volume estimate, which is appropriate for a typical moderately decorated setup. If your tank is heavily scaped or filled with live rock, apply a more conservative factor of 80 to 85 percent to your gross volume for a more accurate net figure. This distinction becomes critically important when medicating a tank — dosing based on gross volume in a heavily aquascaped tank can result in 15 to 20 percent overdosing, which is dangerous for sensitive fish and invertebrates.
How the Aquarium Volume Calculator Works: The Exact Formula
This calculator uses three formulas depending on the tank shape you select. All measurements must be entered in inches, and all calculations use the exact US gallon definition of 231 cubic inches.
Rectangular tanks (including standard and long-style): The formula is straightforward — Volume (gallons) = Length (in) × Width (in) × Height (in) ÷ 231. This is mathematically exact for any rectangular prism. Enter the inside dimensions of the tank, not the outside dimensions. The glass panels add approximately 1/4 inch to 3/8 inch per side depending on tank size, so outside dimensions will give you a slightly inflated volume reading.
Bowfront tanks: Bowfront aquariums have a convex-curved front glass that adds volume beyond what a flat-fronted tank of the same stated dimensions would hold. The exact additional volume depends on the depth of the bow, which varies by manufacturer and tank size. This calculator uses an industry approximation factor of 0.97 applied to the rectangular formula, which accounts for the additional bow volume on typical commercially available bowfront tanks. The actual result is that a bowfront’s calculated volume comes out slightly lower than the rectangular equivalent — this is intentional and correct, because while the bow adds some volume at the front, the stated “width” dimension is typically measured at the middle where the bow is deepest, making the average width effectively less than the stated figure for most of the tank’s length.
Hexagonal tanks: A regular hexagonal tank has six equal sides meeting at 120-degree angles. The base area of a regular hexagon with side length S is calculated as (3 × √3 / 2) × S², which simplifies to approximately 2.598 × S². Multiply this base area by the height, then divide by 231 for US gallons. When you select hexagonal shape, the “Length” input becomes “Side Length” — enter the length of one side of the hexagon, not the diameter or any other measurement.
After calculating gross volume, the calculator automatically computes net usable volume at 90 percent of gross, converts to liters using the exact factor of 3.785411784 liters per US gallon, calculates water weight using 8.34 lbs per gallon for freshwater or 8.55 lbs per gallon for saltwater, and generates a basic stocking estimate using the 1 inch of fish per net gallon guideline as a rough starting point.
Three Real US Aquarium Setup Examples: Calculations in Practice
Example 1: The Classic 55-Gallon Community Tank in an Atlanta Suburb
Jake, a first-time fishkeeper in Marietta, Georgia, picks up what the pet store calls a “55-gallon standard” aquarium kit. The inside measurements of the tank are 47.5 inches long, 12.75 inches wide, and 20.75 inches tall. Using this calculator: 47.5 × 12.75 × 20.75 = 12,566 cubic inches ÷ 231 = 54.4 gross gallons. Net usable volume at 90 percent = 48.96 net gallons, or call it 49 gallons.
Jake fills it as a freshwater community tank with 2 inches of standard pea gravel. He’s planning to stock with tetras, cories, and a pair of German blue rams. Water weight at 90 percent fill: 49 × 8.34 = 408.7 lbs. Glass tank weight for a standard 55-gallon: approximately 78 lbs. Gravel (roughly 50 lbs for a 2-inch bed): 50 lbs. Filter, heater, decor: roughly 25 lbs. Total system weight: approximately 561 lbs. Jake’s stand sits over a floor joist in his living room — wise, since this is a concentrated 561-lb load on roughly 2 square feet of floor.
For water conditioner dosing, Jake uses the actual calculated volume of 49 net gallons rather than the nominal “55 gallons” on the box — this means he uses about 10 percent less product per water change, which adds up significantly over time. His filter is rated for 220 GPH, which provides 4.5 turnovers per hour of the net volume — appropriate for a lightly to moderately stocked community tank.
Example 2: A 125-Gallon Reef Tank in a San Diego Home Office
Maria, an experienced reefer in Chula Vista, California, is setting up a new display tank in her home office. She buys a reef-ready aquarium with inside measurements of 72 inches long, 18 inches wide, and 22 inches tall. The calculation: 72 × 18 × 22 = 28,512 cubic inches ÷ 231 = 123.4 gross gallons. Net usable is 90 percent of 123.4 = 111 gallons. But Maria is building a reef with significant live rock — she’s planning 100 lbs of aquacultured live rock, which at roughly 0.8 lbs per gallon displaced volume will occupy about 7 to 8 gallons of space. Her effective water volume is closer to 103 gallons in the display.
Maria also has a 40-gallon sump. Combined system water volume: approximately 103 gallons display + 35 gallons in the sump (at 90 percent of gross) = 138 gallons total system water. When mixing saltwater to specific gravity 1.025 using Instant Ocean, she needs 0.5 cups of salt mix per gallon, so a full system change would require 138 × 0.5 = 69 cups, or roughly 17.25 lbs of salt mix. Knowing the exact system volume prevents both undersalting (which causes salinity to run low) and oversalting (which is dangerous to corals and invertebrates).
Water weight for the display alone (at 90 percent fill, saltwater): 111 × 8.55 = 949 lbs. Plus the glass tank at approximately 206 lbs for a standard 125-gallon. Plus live rock at 100 lbs, aragonite sand at 60 lbs, equipment at 30 lbs. Display tank total: approximately 1,345 lbs on the stand footprint. Maria had her home office floor reinforced with a steel beam in the crawlspace before placing the tank.
Example 3: A 40-Gallon Breeder Planted Tank in a Brooklyn Apartment
DeShawn is setting up a Dutch-style planted aquarium in his apartment in Crown Heights. He chose the 40-gallon breeder format specifically for its wide footprint — inside dimensions are 35.75 inches long, 17.75 inches wide, and 16.75 inches tall. Calculation: 35.75 × 17.75 × 16.75 = 10,626 cubic inches ÷ 231 = 45.9 gross gallons. It’s called a “40 breeder” but actually holds nearly 46 gross gallons — the extra width gives it significantly more volume than a standard 40-gallon, which is why Dutch-style and nature aquarium aquascapers love this format.
DeShawn is using a 4-inch deep ADA Aquasoil substrate for his planted layout. Aquasoil is lighter than gravel (roughly 60 lbs for a 4-inch bed in this tank vs 90 lbs for gravel), but it occupies significantly more volume — roughly 5 to 6 gallons of displaced volume at 4-inch depth across the full footprint. Net water volume: approximately 46 × 0.85 = 39 gallons. He doses fertilizers using this 39-gallon net figure rather than the nominal 40-gallon or the 46-gallon gross, keeping his macronutrient and micronutrient dosing tight enough for the demanding high-tech planted environment. His CO2 system is tuned to maintain 30 ppm dissolved CO2 in 39 gallons of water, calculated as approximately 1.2 bubbles per second through his ceramic diffuser.
Three Expert Tips for Getting the Most Accurate Aquarium Volume
Tip 1: Always Measure Inside Dimensions, Not Outside
This sounds obvious, but it catches new hobbyists surprisingly often. Commercial aquarium specifications listed by manufacturers and on product pages almost always refer to outside dimensions, because that’s what determines how much floor or shelf space the tank takes up. But when you’re calculating water volume, you need the inside measurements — the space the water actually occupies.
For most standard glass aquariums, the glass panels are 3/16 inch to 1/2 inch thick depending on tank size. On a 55-gallon aquarium with 3/8-inch glass on all sides, the inside dimensions are approximately 3/4 inch narrower than the outside dimensions in both length and width. That’s a volume difference of about 1.5 gallons on a 55-gallon tank — not enormous, but meaningful if you’re dosing medications or mixing saltwater to precise salinity. The easiest approach: use a measuring tape dropped inside the tank for accurate readings. If you don’t have the tank in hand yet, subtract 3/4 inch from the manufacturer’s outside dimensions as a reasonable approximation for standard glass tanks in the 30 to 125-gallon range.
Tip 2: Track Net Volume, Not Gross, for All Dosing and Stocking Calculations
The 90 percent net volume figure this calculator provides is a useful starting approximation, but your specific setup may differ meaningfully. Here’s a better approach for any tank you care about maintaining precisely: once your tank is fully set up with all substrate, hardscape, and equipment in place, fill it using measured containers. Count the gallons as you go. That number — the actual water you poured in — is your real net volume. Write it on a piece of masking tape and stick it to the inside of your stand door. Use that number for every dosing, medication, and treatment calculation you ever do on that tank.
This matters most for reef keepers, planted tank enthusiasts, and anyone medicating a tank. A reef aquarist using Seachem Stability at the recommended rate of 1 cap per 50 gallons who uses gross instead of net volume may end up using 15 to 20 percent more product than necessary every single time. A planted tank hobbyist dosing ThriveAll by NilocG or similar dry fertilizer using gross volume in a heavily aquascaped Dutch tank can push nutrient levels well above target. The math is simple; the habit of measuring actual net volume accurately is where most people save both money and problems.
Tip 3: Calculate Weight Before You Fill the Tank
Water weighs 8.34 lbs per gallon. That fact looks innocent until you’re staring at a 125-gallon aquarium that weighs over 1,300 lbs sitting on a floor you haven’t reinforced. The time to think about floor load is before the tank is full — not after. Once a large aquarium is in place and filled, you have essentially no practical options if the floor turns out to be inadequate. Moving a full 75-gallon or larger tank is a multi-person project that requires draining most of the water first, stressing the fish, and risking cracking the silicone seams.
Standard US residential floors built to International Residential Code specifications are designed for a minimum live load of 40 lbs per square foot in living areas. But this is a distributed load standard, not a point-load standard. An aquarium stand distributes the weight of a 75-gallon system (approximately 800 to 900 lbs) across a stand footprint of roughly 13 x 24 inches, or about 2.17 square feet. That works out to 370 to 415 lbs per square foot — nearly ten times the floor rating. The tank doesn’t cause a collapse in most cases because the stand spreads its feet over floor joists rather than pushing between them. But you need to be intentional about this. Use the Aquarium Weight Calculator to confirm floor safety before filling any tank 55 gallons or larger, and always position large tanks so the stand spans perpendicular to the floor joists, or better yet, directly over a load-bearing wall below.
What Do Fishkeepers Most Often Get Wrong About Tank Volume?
Multiply Length × Width × Height (all in inches), then divide by 231. The number 231 is not approximate — one US gallon is legally defined as exactly 231 cubic inches. Example: 48 × 13 × 20 = 12,480 cubic inches ÷ 231 = 54.0 gallons. Always use inside tank dimensions, not outside. The glass panels add about 1/2 to 3/4 inch to outside dimensions compared to inside, and measuring the outside will inflate your gallon count by 1 to 3 gallons depending on tank size.
A full 55-gallon freshwater aquarium weighs approximately 625 to 700 lbs total. The breakdown: water alone at 90 percent fill = approximately 49 gallons × 8.34 lbs/gal = 408 lbs. Empty glass tank = approximately 78 lbs. Gravel (2-inch bed) = approximately 50 lbs. Filter, heater, decor, and stand = approximately 75 to 100 lbs. If you run a saltwater 55-gallon, substitute 8.55 lbs per gallon for water weight and add the weight of live rock and sand accordingly. The total will climb past 700 lbs easily for a reef setup.
A 48 × 12 × 18 inch aquarium holds 48 × 12 × 18 = 10,368 cubic inches ÷ 231 = 44.9 gross US gallons. Net usable volume at 90 percent = approximately 40.4 gallons. Water weight freshwater: approximately 337 lbs. This is a common format sold as a “40-gallon long” or in some cases a “40-gallon breeder” depending on manufacturer, though true breeders are typically a bit wider. The long footprint makes this format popular for cichlids, goldfish, and bottom-dwelling species that benefit from horizontal swimming space.
Gross volume is the total mathematical volume of the empty tank based on inside dimensions. Net usable volume is the actual water volume after subtracting space occupied by substrate (gravel, sand, soil), decorations, live rock, equipment, and the air gap at the top where you don’t fill to the rim. Net is typically 85 to 95 percent of gross for most setups. This calculator uses 90 percent as a typical estimate. Always use net volume for stocking density calculations, medication dosing, and salt mixing. Using gross volume for these purposes results in systematic under-concentration of treatments and additives.
Freshwater at typical aquarium temperatures (68 to 78°F) weighs 8.34 lbs per US gallon. Saltwater at marine aquarium salinity (specific gravity 1.025, approximately 35 ppt salt) weighs approximately 8.55 lbs per US gallon. This 2.5 percent difference adds up significantly on large tanks — a 150-gallon reef holds approximately 31.5 more lbs of water than a 150-gallon freshwater tank. When calculating floor loads and total system weight for large marine tanks, always use the saltwater figure of 8.55 lbs per gallon, not the freshwater figure.
For a regular hexagonal aquarium where all six sides are equal length, the base area formula is: (3 × √3 / 2) × S², where S is the side length in inches. This simplifies to approximately 2.598 × S². Multiply that base area by the height in inches to get cubic inches, then divide by 231 for US gallons. Example: a hexagonal tank with 12-inch sides and 24-inch height has base area = 2.598 × 144 = 374.1 sq in. Volume = 374.1 × 24 = 8,978 cubic inches ÷ 231 = 38.9 gallons. Select “Hexagonal” in this calculator and enter the side length and height — the formula runs automatically.
One US gallon equals exactly 3.785411784 liters. To convert, multiply your gallon figure by 3.785. A 55-gallon aquarium holds approximately 55 × 3.785 = 208.2 liters. Note carefully: the UK Imperial gallon equals 4.546 liters, which is significantly different. If you are purchasing a tank that was manufactured or specified in a European or UK context, confirm whether the liter rating is being converted from US or Imperial gallons. Most US-sold aquarium products (filter ratings, medication dosages, salt mix instructions) are calibrated to US gallons specifically.
Two formats are commonly sold as “20-gallon”: the 20-gallon high (24 × 12 × 16 inches inside) calculates to 24 × 12 × 16 ÷ 231 = 19.9 gallons — very close to the label. The 20-gallon long (30 × 12 × 12 inches inside) calculates to 30 × 12 × 12 ÷ 231 = 18.7 gallons — slightly less than the nominal 20-gallon despite the same label. The “long” format provides more surface area for gas exchange (important for stocking density) and a better footprint for most fish, which is why many experienced aquarists prefer it over the “high” format despite the slightly lower actual volume.
The 20-gallon long, 29-gallon, or 40-gallon breeder are the most recommended starting sizes for US beginners. The common “starter kit” advice to begin with a 5 or 10-gallon is actually counterproductive for most new hobbyists. Smaller tanks are more chemically volatile, require more frequent maintenance, are less forgiving of mistakes, and limit the number and variety of fish you can keep. A 29-gallon or 40-gallon breeder provides the stability to survive early mistakes while keeping cost and space requirements manageable. The 55-gallon is the most common tank in the US hobby and is widely regarded as the sweet spot of size, cost, and manageability for community freshwater setups.
Bowfront tanks are challenging to calculate precisely because the additional volume from the curved front glass varies significantly by manufacturer and model. This calculator uses a factor of 0.97 applied to the rectangular formula, which is a conservative industry approximation. For the most accurate measurement of a specific bowfront you already own, the definitive method is to fill it with a known quantity of water using a measuring container, counting each gallon or liter as you pour. This eliminates all estimation and gives you the exact net volume for your specific tank, which you can then use for all dosing and equipment calculations going forward.
The 1 inch of adult fish body length per gallon of net tank volume is the most widely cited stocking guideline in the freshwater hobby. It was developed as a rough rule of thumb for small community fish and is widely considered outdated and oversimplified by modern fishkeeping standards. Its main problems: it doesn’t account for fish waste production rates (a 6-inch goldfish produces far more waste than six 1-inch neon tetras); it doesn’t account for territorial behavior; it ignores the difference between a slim-bodied tetra and a wide-bodied goldfish; and it breaks down entirely for large fish (no sane aquarist thinks a 12-inch oscar belongs in a 12-gallon tank). Use it only as a very rough starting point and research the specific needs of each species you intend to keep.
A standard US household faucet or garden hose flows at approximately 2 gallons per minute. At that rate: a 10-gallon tank takes about 5 minutes; a 29-gallon takes 15 minutes; a 55-gallon takes about 28 minutes; and a 100-gallon takes roughly 50 minutes. A Python No Spill Clean and Fill system, which connects your aquarium to a faucet, can speed this process while also allowing you to add dechlorinator directly to the stream. For very large tanks (100 gallons and up), many aquarists use a dedicated submersible pump and hose to transfer pre-mixed water from storage containers rather than filling directly from tap and treating in the tank, especially for saltwater systems where precise salinity control is essential.
The standard guideline is to filter the entire tank water volume 4 to 6 times per hour for typical freshwater community tanks, and 8 to 10 times per hour for saltwater fish-only and reef tanks. For a 55-gallon freshwater community tank (net volume approximately 49 gallons), you need a filter rated for at least 200 to 295 GPH. For a reef tank of the same gross size, aim for 400 to 490 GPH through the sump return pump alone. Important caveat: filter ratings on packaging are measured with clean filter media under ideal conditions. Real-world flow through dirty or biological media is typically 25 to 50 percent lower. Always choose a filter rated for a tank significantly larger than yours, or run two medium-sized filters for redundancy on larger setups.
The International Residential Code (IRC) sets a minimum residential floor live load of 40 lbs per square foot. But this is a distributed load standard, and aquarium stands create concentrated point loads that far exceed this. A 75-gallon aquarium system weighing approximately 800 to 900 lbs on a 13 x 25-inch stand footprint applies roughly 370 to 415 lbs per square foot — nearly 10 times the minimum floor rating. The critical factor is whether the stand legs or base sit over floor joists, which transfer the load down to the beam and foundation, vs between joists where only the subfloor decking absorbs the weight. For any tank 55 gallons and larger, use the Aquarium Weight Calculator and position the tank so the stand spans perpendicular to joists or over a load-bearing wall. For 100 gallons and larger on a second floor, consult a structural engineer.
The nitrogen cycle is the biological process by which beneficial bacteria (Nitrosomonas and Nitrospira species) colonize your filter media and substrate, converting toxic fish waste products (ammonia) first to less-toxic nitrite, then to relatively harmless nitrate. A new tank needs 4 to 8 weeks to fully establish this bacterial colony, during which ammonia and nitrite can spike to lethal levels — this is why adding fish to an uncycled tank kills them. Tank volume directly affects cycle stability: in a 10-gallon tank, one extra feeding can push ammonia to 4 ppm within 24 hours; in a 55-gallon, the same overfeeding might raise ammonia only to 0.5 ppm, giving your bacteria time to process it before it reaches dangerous levels. Larger water volume is biological insurance. Test parameters daily during cycling, perform water changes when ammonia or nitrite exceed 0.5 ppm, and don’t add fish until ammonia and nitrite both read zero and nitrate is climbing.
A standard freshwater gravel bed of 1 to 2 inches deep requires approximately 1.5 lbs of gravel per gallon of gross tank volume. A 55-gallon tank with a 1.5-inch gravel bed needs roughly 80 to 85 lbs of gravel. That gravel also displaces approximately 2 to 3 gallons of water volume in the 55-gallon tank, contributing to the reduction from gross to net volume. Planted tanks using nutrient-rich substrates (ADA Aquasoil, Eco-Complete) typically require a much deeper bed of 3 to 4 inches and are applied at lower density — follow substrate-specific package instructions rather than the general gravel formula, as these materials behave differently. Deep substrates like this can displace 4 to 6 gallons in a standard 55-gallon tank, making the difference between gross and net volume more significant than in a simple gravel setup.
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Legal Disclaimer and Editorial Transparency
Editorial standards: Content on this page is reviewed for accuracy against published US aquarium industry standards, the International Residential Code (IRC), and widely accepted aquarium husbandry practices. Stocking density guidance uses the 1-inch-per-gallon rule as a rough starting point only — it is not a comprehensive fish stocking recommendation and does not replace species-specific research. Floor load guidance is informational; consult a licensed structural engineer before placing large aquariums, particularly on upper floors or in older construction. This tool is for informational and planning purposes only and does not constitute professional engineering, veterinary, or fish husbandry advice.