Will your fish actually fit?
Check your stocking level before you buy. Freshwater, method-transparent, no guesswork.
Your fish
Not in the list? by adult length (cm).
Check your stocking level before you buy. Freshwater, method-transparent, no guesswork.
Under 100% doesn’t mean “add more until it’s full.” The gauge is a planning guide, not a licence. A tank at 86% with a light filter and infrequent water changes can be under more strain than one at 110% that’s heavily planted and over-filtered. What matters is the balance between what your fish produce (waste, the “bioload”) and what your system removes (filtration, plants, water changes).
Treat the bands as a starting point, not a verdict. Under 60% you have real room to add fish, or you can leave it light and enjoy the lower maintenance. 60–85% is a healthy working load for most community setups. 85–100% is well stocked: fine to run long-term, but any further addition should be small and slow. Anything past 100% needs a change — more filtration, fewer fish, or a bigger tank — before you buy anything else. Past 115%, treat it as urgent even in an established tank.
Separately, watch for hard-stop warnings. These override the percentage because they’re not about bioload math at all — they’re about a species that physically won’t thrive in the tank you have, or a schooling fish kept in too small a group to feel safe. A goldfish in a 40-litre tank can show a fine-looking percentage and still be a bad idea, because goldfish need floor space and years of growth room a small footprint can’t give them.
Each fish on your list gets a bioload score: its adult length in centimetres, multiplied by a species bioload factor. The factor is a modelling judgement, not a lab measurement — it scales how messy a fish is for its size. A 10cm plume-tailed goldfish produces far more waste than a 10cm cluster of neon tetras, so goldfish carry a higher factor and tetras a lower one. Multiply that score by how many of that fish you’re keeping, and add every row together to get your total bioload.
Your tank’s capacity, in the same units, is its volume in litres multiplied by a filtration multiplier. Standard filtration holds the line at roughly one bioload-unit-centimetre per litre. Over-filtered tanks (canister filter sized up, or extra biological media) raise that ceiling by 30%. Under-filtered tanks (an undersized hang-on-back, or a heavily stocked tank with no secondary filtration) lower it by 30%. Divide your total bioload by that capacity and multiply by 100, and that’s your stocking percentage.
This is a hybrid method on purpose. A pure bioload formula misses physical constraints — it would happily tell you thirty neon-sized fish fit in a nano tank if the math balances, when in reality there’s no room to swim and no minimum-tank-size check. So the calculator also runs a separate sanity check against each species’ documented minimum tank size and, for schoolers, its minimum group size. Either one failing produces a hard-stop warning regardless of what the percentage says.
The inch-per-gallon rule says one inch of adult fish needs one gallon of water, full stop. It’s easy to remember and it ignores nearly everything that actually determines whether a tank is overstocked. It treats a slim, low-waste inch of neon tetra the same as an inch of fancy goldfish, which produces several times more ammonia. It ignores filtration entirely, so a tank with a filter rated for triple its volume gets the same allowance as one running bare minimum equipment. And it ignores body shape: a deep-bodied oscar and a slender pencilfish of the same length displace very different amounts of waste and water.
It also breaks down completely at the small end and the large end. A single 2-inch betta in a 2-gallon bowl passes the rule on paper and is still a welfare problem — bettas need more swimming space and stable water than that math implies. At the other end, a 12-inch common goldfish “fits” a 12-gallon tank by the rule, but goldfish are exceptionally messy, grow for years, and need roughly five times that volume in practice. The rule isn’t wrong because it’s simple. It’s wrong because it drops every variable that actually drives water quality.
Adding everything on day one.A brand-new tank has no established bacteria colony to process waste. Stocking it fully before it’s cycled causes an ammonia spike that burns gills and kills fish within days. Add fish gradually over weeks, testing water after each addition, even if the calculator says you’re under 100%.
Buying for adult size at juvenile size.A 3cm common pleco in the shop tank looks harmless. It can reach 45cm. Always stock and plan around the adult size a species reaches, not the size it is when you buy it — this calculator uses adult figures for exactly that reason.
Under-filtering for the bioload, not the tank size. Filter ratings assume an average bioload. A tank full of messy fish needs more filtration than the same size tank of shrimp and nano fish, even though the volume is identical.
Ignoring schooling minimums. A single neon tetra, or three where six is the minimum, spends its time stressed and hiding rather than behaving naturally. Chronic stress suppresses the immune system and is a common, invisible cause of disease outbreaks in otherwise clean water.
Skipping water changes because the filter “handles it.”Filtration processes ammonia and nitrite into nitrate. It doesn’t remove nitrate, dissolved organics, or replenish trace minerals. Regular partial water changes are still required at any stocking level.
Cycling is growing the bacteria colony that converts fish waste (ammonia) into less toxic compounds. A tank isn’t safely stockable at its calculated capacity until it’s cycled, typically 4–6 weeks with fishless cycling methods.
No. Add a few at a time, spaced by a week or more, and test ammonia and nitrite after each addition. The bacteria colony needs to grow to match a larger bioload; it doesn’t scale instantly.
Within reason, yes — more biological filter media processes waste faster, which is why the over-filtered option raises the ceiling in this calculator. It doesn’t override a minimum tank size or schooling requirement, which are physical, not chemical, constraints.
Use “Add a custom fish” and enter its adult length. The calculator will run the bioload math on that figure, though it can’t apply a species-specific minimum tank size or schooling check for fish it doesn’t have data on — look those up separately for anything unusual.
No. This is freshwater only. Reef systems have entirely different bioload, filtration, and stocking dynamics.