Fish Tank Stocking Calculator

Find out how heavily your aquarium is stocked before you add another fish. Enter your tank volume, add the fish you keep or plan to keep, and this fish tank stocking calculator scores the tank on adult size and waste output — not the one inch per gallon rule.

40 common species Adult size & bioload Filtration-adjusted Always free

🐠 Aquarium Stocking Calculator

Results update as you add fish. Every species is scored at its adult size, because that is the size your filter will be dealing with in a year. The same method works as a fish tank stocking calculator for a 5 gallon betta tank or a 125 gallon cichlid tank.

Use the actual water volume if you know it. Substrate and decor usually displace 10 to 15 percent of the figure on the box — the tank size calculator works it out from the dimensions.
Filtration decides how much waste the tank can process, not how much room the fish have. The calculator adjusts capacity by about 15 percent either side of a correctly rated filter.
Fish in the tank

    Common tank sizes

    Jump straight to a standard aquarium size. These are US gallon figures for the tank as sold — switch the unit above if you work in litres.

    How it works

    How to Use the Fish Tank Stocking Calculator

    Four inputs, one honest answer about whether the tank can carry the fish you want.

    1

    Enter tank volume

    Use the real water volume if you can. A "20 gallon" tank with gravel, rock and a filter intake holds around 17 gallons of water.

    2

    Describe the filter

    Be honest. A cycled canister rated for the tank is "good". A sponge filter on a 40 gallon is "basic" no matter what the shop said.

    3

    Add your fish

    Pick each species and how many. Include the fish already in the tank as well as the ones you are thinking of buying.

    4

    Read the percentage

    Under 90 percent leaves room. Over 100 means the tank is full at adult size, whatever the fish look like today.

    Fish tank stocking calculator principle diagram: two aquariums holding the same six inches of fish — four neon tetras at 4 gallons of load against one fancy goldfish at 12 — beside the four-step calculation worked for a 20 gallon tank: capacity 20 gallons of load, fish load 8 neon tetras plus 6 corydoras equals 17, stocking 85 percent, well stocked, then the minimum tank and group size checks and a stocking meter.
    How the calculation works. Left: why the tool scores each species by gallons of load rather than inches — the same six inches of fish can be three times the waste. Right: the four steps the calculator runs, worked for a 20 gallon tank with 8 neon tetras and 6 corydoras. Open full-size diagram.

    How Many Fish Per Gallon?

    The rule everyone learns first is one inch of fish per gallon of water. It is easy to remember and it is roughly right for the fish it was written about — small, slim community fish like tetras, rasboras and guppies in a tank with an ordinary filter. Ten neon tetras in a 10 gallon tank is about where that rule lands, and it is a reasonable stocking level.

    The rule breaks the moment the fish get bigger or heavier. A fancy goldfish at 6 inches has perhaps ten times the body mass of four 1.5 inch tetras, eats far more, and produces waste in proportion. The inch rule counts them the same. Worse, it says nothing about swimming room: one 10 inch oscar "fits" a 10 gallon tank by the arithmetic, and no oscar keeper would put one in anything under 75.

    Stocking is really three separate limits:

    • Biological load. Ammonia and nitrite have to be processed as fast as the fish produce them. This is the limit that filtration and plants can move.
    • Nitrate and water changes. The end product of the nitrogen cycle builds up between water changes in proportion to the bioload. More fish means bigger or more frequent changes.
    • Physical space. Schooling fish need a length of tank to swim, bottom dwellers need floor area, and territorial fish need space to avoid each other. Nothing but a bigger tank moves this limit.

    This calculator handles the first two by giving every species a gallons-per-fish figure that reflects its adult size and how much waste it produces, then adjusts that for your filter and plants. It handles the third by checking each species against a minimum tank size that no amount of filtration overrides.

    Working figures by fish size

    If you want a quick planning number without the calculator, these are the gallons per fish the tool works from, for a tank with a correctly rated filter.

    Approximate gallons of tank per adult fish, by body size and type, in a community tank with a filter rated for the volume.
    Type of fishAdult sizeGallons per fishExamples
    Nano schooling fishUnder 1 in0.6 – 0.8Chili rasbora, ember tetra, pygmy cory
    Small schooling fish1 – 2 in1 – 1.5Neon tetra, harlequin rasbora, guppy, zebra danio
    Small centrepiece or bottom fish2 – 3 in2 – 3Betta, platy, corydoras, German blue ram
    Medium community fish3 – 5 in3 – 6Molly, dwarf gourami, kribensis, rainbowfish
    Large or heavy-bodied fish5 – 7 in8 – 12Angelfish, discus, bristlenose pleco, fancy goldfish
    Very large fish10 in and over40 – 60Oscar, common goldfish, common pleco
    InvertebratesAny0.15 – 1.5Cherry shrimp, nerite snail, mystery snail

    How Many Fish in a 10 Gallon Tank?

    A 10 gallon tank comfortably holds one small school of eight nano or small fish, or a single centrepiece fish with a small school around it. It does not hold goldfish, angelfish, any gourami bigger than a honey gourami, or anything that reaches 4 inches, regardless of how small the fish are when you buy them.

    Example stockings for a 10 US gallon (38 litre) tank with a correctly rated filter, scored by this calculator.
    StockingStocking levelNotes
    8 neon tetras80%The classic 10 gallon school. Room for a few shrimp or a nerite snail.
    1 betta + 6 chili rasboras66%Works if the betta is placid. Add the rasboras first, the betta last.
    6 male guppies90%Males only, or the fry will overstock the tank within months.
    6 pygmy corydoras + 8 ember tetras96%Full. Both species stay under an inch, so it looks emptier than it is.
    1 honey gourami + 8 ember tetras73%Calm centrepiece with a nano school. Heavy planting suits both.
    1 fancy goldfish120% — not suitableNeeds 20 gallons minimum. Goldfish are the most common 10 gallon mistake.

    Whatever you choose, stock a 10 gallon in two stages a few weeks apart. Small volumes swing fast, and a newly cycled filter needs time to grow into a full load. If a fish on your list comes out as "needs a larger tank" here, the answer is a larger tank — the tank size calculator will tell you how large.

    Fish Tank Stocking Chart by Tank Size

    A well stocked example for each standard aquarium size, assuming a filter rated for the tank and a light to moderate amount of plants. Each of these lands between 80 and 96 percent on the calculator — full, with a little margin.

    Example community stockings by tank size, scored between 80 and 96 percent by this calculator at "good" filtration.
    Tank sizeLitresExample stockingLevel
    5 US gal191 betta + a cherry shrimp colony90%
    10 US gal388 neon tetras + 2 nerite snails86%
    20 US gal long768 neon tetras + 6 bronze corydoras85%
    29 US gal1101 dwarf gourami + 10 harlequin rasboras + 6 corydoras86%
    40 US gal breeder1511 pearl gourami + 12 rummy nose tetras + 8 corydoras90%
    55 US gal2082 angelfish + 10 cardinal tetras + 8 corydoras + 1 bristlenose96%
    75 US gal2846 Boesemani rainbowfish + 12 cardinals + 8 corydoras + 1 bristlenose88%
    125 US gal4731 oscar + 1 common pleco92%

    Tank shape matters as much as volume, and a fish tank stocking calculator cannot see shape. A 20 gallon long has twice the floor space of a 20 gallon high and suits schooling fish and corydoras far better; a 40 gallon breeder is the best-shaped tank on this list for the money. These are ceilings for the layout shown, not shopping lists — every combination still needs checking for temperament and water chemistry in the fish compatibility checker.

    Signs Your Fish Tank Is Overstocked

    An overstocked aquarium rarely fails all at once. It shows up as a tank that is harder to keep stable than it should be.

    • Nitrate climbing fast between water changes. If a 25 percent weekly change no longer keeps nitrate under 20 to 40 ppm, the bioload has outgrown the routine.
    • Ammonia or nitrite that will not stay at zero. A mature filter on a correctly stocked tank reads zero for both. Persistent traces mean the filter cannot keep up.
    • Fish at the surface in the morning. Dissolved oxygen is lowest just before the lights come on. Gasping at dawn is the classic low-oxygen signature of a crowded tank.
    • Algae you cannot get on top of. More fish means more dissolved nutrients, whatever the lighting schedule.
    • Constant chasing and torn fins. Crowding removes the space fish use to avoid each other, so aggression that would be harmless in a bigger tank becomes constant.
    • Recurring ich, fin rot and unexplained losses. Chronic stress suppresses the immune response and keeps parasites circulating.

    If you are seeing these, test first. The water parameter checker will tell you whether ammonia, nitrite, nitrate or pH is the immediate problem, but if the readings keep drifting back after every water change, the fix is fewer fish or more water rather than more chemistry.

    Fish tank stocking meter from 0 to over 150 percent in five bands — lightly, well, fully, heavily and overstocked — with the water change each needs and a 20 gallon example for each: 8 neon tetras at 40 percent, 8 neons and 6 corydoras at 85, plus a dwarf gourami at 105, 10 neons with corydoras and gourami at 115, and 8 neons, corydoras and an angelfish at 135 percent where the angelfish also fails the 29 gallon minimum tank.
    What each band of the result means, and how the same 20 gallon tank moves through them as fish are added. Well stocked is the target; anything over 100 percent is full at adult size. Open full-size diagram.

    Stock in stages

    Even a tank that scores well here should not be filled in one afternoon. A cycled filter carries the bacteria for the load it was cycled with, and it takes a week or two to grow into a bigger one. Add a third to a half of the planned fish, wait two to three weeks, test, then add the next group. Schooling fish should be added as a complete group, so plan the stages around whole schools rather than splitting one.

    FAQ

    Frequently Asked Questions

    Stocking questions, answered with the numbers behind them.

    There is no single number, because a neon tetra and a goldfish are not the same fish. As a planning figure, small slim-bodied community fish under 2 inches work out at roughly one fish per gallon, medium fish of 3 to 4 inches at one per 3 to 5 gallons, and heavy-bodied fish such as fancy goldfish at one per 10 to 12 gallons. This calculator assigns each species a gallons-per-fish figure based on its adult size and waste output, then compares the total against your tank.
    Around eight small schooling fish such as neon tetras, ember tetras or chili rasboras, or a single betta with six small rasboras, or six pygmy corydoras with a small school of tetras. A 10 gallon tank is too small for goldfish, angelfish, most gouramis larger than a honey gourami, and any fish that reaches 4 inches. Stock it in two or three stages a few weeks apart so the filter can catch up.
    A well stocked 20 gallon community tank is typically one school of eight to ten small tetras or rasboras plus a group of six corydoras, or a centrepiece fish such as a dwarf gourami with a school of ten small fish. A 20 gallon long is a better shape than a 20 gallon high for the same volume, because the extra floor space gives bottom dwellers and schooling fish more room to move.
    Only for small, slim fish in a tank with ordinary filtration. It undercounts the waste of deep-bodied fish — a 6 inch fancy goldfish produces many times the waste of four 1.5 inch tetras — and it ignores swimming room, so it would allow a 10 inch oscar in a 10 gallon tank. This calculator uses adult size and a bioload factor per species instead, and shows the plain inch-per-gallon figure alongside so you can see where the two disagree.
    Up to a point. An oversized canister filter or sump processes more ammonia and nitrite, and live plants take up some of the nitrogen the fish produce, so a heavily filtered and planted tank can carry around 20 to 30 percent more than the same tank on a small sponge filter. What filtration cannot change is swimming space and territory, which is why the calculator also checks each species against a minimum tank size that no filter overrides.
    The clearest signs are nitrate that climbs quickly between water changes, ammonia or nitrite that will not stay at zero on a mature filter, fish gasping at the surface in the morning, algae you cannot get on top of, and more frequent disease outbreaks. If your stocking level here comes out above 100 percent and you are seeing any of those, the fix is fewer fish or a larger tank rather than more chemicals.
    Barely. Dwarf shrimp and nerite snails produce very little waste, and a colony of cherry shrimp adds less bioload than a single small tetra. Larger snails such as mystery snails and apple snails do count, roughly as a small fish each. The calculator includes common invertebrates with a low load figure so a planted shrimp tank comes out realistically rather than showing as empty.
    The usual guideline is 20 gallons for the first fancy goldfish and 10 gallons for each additional one, so two fancies in a 30 gallon tank or three in a 40. Single-tail goldfish such as commons and comets grow to 12 inches and are pond fish in practice — plan on 75 gallons or more for a small group. Goldfish are the clearest example of why the inch-per-gallon rule fails: they are heavy-bodied, messy and long-lived.
    Free tools

    Related Aquarium Tools

    Stocking is one part of the plan. These cover the rest of the tank.

    Fish Tank Size Calculator

    Work out the real water volume from your tank dimensions, after substrate and decor.

    Fish Compatibility Checker

    Check that the fish on your list get along — temperament, temperature and pH.

    Fish Feeding Calculator

    How much and how often to feed the stocking level you have settled on.

    Water Parameter Checker

    Check whether the readings say your stocking level has outgrown the filter.

    Planning a pond rather than a tank? The koi pond stocking calculator uses pond-specific figures. For a full setup checklist from cycling to first fish, start with the aquarium planner, and the tank setup guide covers the order to do it in.

    Methodology and accuracy

    Each species carries a gallons-per-fish figure that reflects its adult length and body mass, and therefore its waste output, in a community tank with a filter rated for the volume. The figures range from 0.15 gallons for a cherry shrimp through 1 gallon for a neon tetra, 3 for a betta, 10 for an angelfish and 12 for a fancy goldfish, to 55 to 60 for an oscar or common pleco. They are planning conventions drawn from widely used hobby guidelines rather than measured carrying capacities, and sources vary either side of them.

    Stocking level is the sum of gallons-per-fish across all fish, divided by the tank volume after the filtration and plant adjustments, expressed as a percentage. Filtration multiplies capacity by 0.85 for a basic filter, 1.0 for a correctly rated cycled filter, and 1.15 for an oversized canister or sump. Live plants multiply it by 1.05 for a moderately planted tank and 1.15 for a heavily planted one. The adjustments are deliberately small: filtration and plants change the biological limit, not the amount of swimming space.

    Minimum tank sizes and minimum group sizes are checked separately and are not affected by filtration. A species that needs 20 gallons is flagged in any tank under 20 gallons however lightly stocked it is. The one inch per gallon cross-check sums adult lengths and compares them to the raw volume; it is shown because it is the figure most people know, not because it is the better guide.

    Volume conversions are exact: 1 US gallon = 3.785411784 litres, 1 imperial gallon = 4.54609 litres. The calculator does not check temperament, temperature or water chemistry compatibility — use the fish compatibility checker for that. It estimates capacity; it is not a substitute for testing your own water.