Walk into any aquatic shop and ask how many fish you can keep in a 75-litre tank, and someone will give you a version of the "one inch per gallon" rule. The 75-litre tank holds about 20 US gallons, so you can have 20 inches of fish — twenty 1-inch tetras, ten 2-inch corys, or one big 20-inch knifefish, take your pick.
The first two are roughly defensible. The third would kill the fish in a week.
The problem with inches-per-gallon is that it treats all "fish-inches" as interchangeable. They aren't. A 5 cm cardinal tetra and a 5 cm fancy goldfish produce wildly different waste loads, need wildly different amounts of swimming room, and tolerate wildly different water parameters. A rule that says they're equivalent is a rule that's wrong on the only axis that matters.
This article is about the math that actually works — the four variables I check before adding any species to any tank, and how I weigh them against each other.
What "stocking" actually means
Before the math, a definition. Stocking is the question of how many fish, of which species, can live in a tank long-term without compromising their health, behaviour, or lifespan. It is not the question of how many fish you can cram into a tank before someone visibly dies. The two are different by a factor of three or four.
A well-stocked tank looks under-stocked to a beginner. Adult fish swim freely, exhibit natural behaviours (schooling, foraging, mild territorial display), and don't show stress markers like clamped fins, hovering near the surface, or unusual aggression. An over-stocked tank can look fine for months — the fish appear active and feeding — and then collapse all at once when a single equipment failure tips a system that was running with no margin.
The goal of stocking math is to give you a margin. Everything below is the calculation of that margin.
Variable 1: Bioload (the actual driver)
Bioload is the rate at which a fish produces nitrogenous waste — primarily ammonia from gill respiration and faeces. Your filter and the bacteria within it convert ammonia → nitrite → nitrate. Plants and water changes remove the nitrate. The whole loop has a finite throughput, and bioload is the input.
Two facts that the inch-per-gallon rule ignores:
- Bioload scales roughly with body mass, not body length. A fish that is twice as long is roughly eight times the volume — so eight times the mass, and very roughly eight times the bioload. This is why a 20 cm goldfish is not "equivalent" to ten 2 cm tetras in terms of waste production. The goldfish is more like 100 tetras.
- Different species have different metabolic rates. A goldfish or any cyprinid eats more, defecates more, and produces more ammonia per gram of body weight than a comparable-sized tetra or characin. Carnivores produce more concentrated waste than detritivores. Cold-water fish are slightly less metabolically active than tropicals at higher temperatures.
So the first thing I do for any prospective species is look up its adult size (not the size at which the shop sells them — adult), then mentally cube it to estimate relative bioload. A 10 cm fish is eight times the bioload of a 5 cm fish, all else being equal. A 20 cm fish is sixty-four times.
This is why the rule of thumb "one fancy goldfish per 75 litres, plus 40 litres for each additional" exists, and why it sounds absurd to someone who's just paid £4 for a fish the size of a 10p coin. They grow. To 15–20 cm. And they eat the entire time.
Variable 2: Surface area, not volume
Oxygen enters the water at the surface — through the air-water interface and through any agitation that breaks that interface (filter outflow, air stones, surface ripple). It does not enter through the sides of the tank, however large the tank is. A tall, narrow column of water has a small surface and is therefore oxygen-limited even if its total volume is generous.
Two tanks with identical 100-litre volumes:
| Tank | Dimensions (L × W × H) | Surface area | Effective stocking |
|---|---|---|---|
| Long & shallow | 100 × 40 × 25 cm | 4,000 cm² | Higher |
| Tall & narrow | 40 × 40 × 62 cm | 1,600 cm² | Lower |
The long, shallow tank has 2.5× the surface area. It can support significantly more active swimmers, and it's far more forgiving when the air pump or filter quits in the middle of the night. The tall, narrow tank looks more dramatic on a shelf — and many "designer" cube and column aquariums fall into this trap — but it stocks more conservatively.
The functional rule I use: for surface-active or oxygen-hungry species (most cyprinids, most cichlids, anything that swims fast), I want at least 25 cm² of surface area per cm of adult fish length. For low-activity species (bettas, gouramis with labyrinth organs, slow-moving tetras) I'll go down to 15 cm² per cm.
Variable 3: Swimming style and territory
This is where pure maths runs out and species-specific knowledge takes over.
A 5 cm zebra danio and a 5 cm betta are the same length. The danio is built for continuous, fast, lap-swimming and gets visibly stressed in any tank shorter than 60 cm — it'll spend its days bouncing back and forth from one end to the other. The betta will happily occupy a 30 cm tank, drift around the plants, and ignore most of the available volume.
Some general patterns I've found reliable across freshwater species:
- Schooling species need a minimum group size. Most tetras, rasboras, danios, and corys want 6+ of their own kind. A tank that "fits" four cardinals on stocking math is not enough — you need a tank that fits six. Round up, always.
- Fast swimmers need length, not just volume. Tank length should be at least 6× the body length of the most active swimmer for any tropical community species, and 10× for active cyprinids like danios and barbs.
- Territorial species need defended space. Most cichlids will claim a "territory" of around 30–60 cm of tank length per pair. Two pairs in a 75 cm tank fight constantly. The same two pairs in a 120 cm tank ignore each other.
- Bottom dwellers and mid-water swimmers stack. A community of corys (bottom), tetras (mid), and hatchetfish (top) uses the available water column more efficiently than three mid-water schools competing for the same space.
Variable 4: Filter throughput and water changes
Stocking is a function of how much waste your tank can process, not just how much it can hold. A filter rated for 4× tank turnover per hour, generously sized media, and a 25% weekly water change will support significantly more bioload than a barely-adequate filter and a fortnightly change.
I aim for the following baseline whenever I'm stocking a community tank:
- Filter throughput: 5–8× tank volume per hour (rated, before media compaction). For a 200-litre tank, that means a filter rated 1000–1600 L/h.
- Biological media volume: roughly 1% of tank volume. Ceramic rings, sintered glass, or mature sponge — the surface area for nitrifying bacteria.
- Water change schedule: 25% weekly. More if heavily stocked. Some keepers go larger and less frequent (50% fortnightly); the maths works out similarly for nitrate but worse for trace stability.
If you cannot meet those baseline numbers, your stocking ceiling drops proportionally. A tank with an undersized filter and a busy schedule that can only accommodate monthly water changes should be stocked at perhaps 50% of what an identical, properly equipped tank would handle.
Putting it together: the four-stage check
Here is the procedure I run mentally before adding any new species or making any new stocking plan. It takes about ten minutes per tank.
Stage 1: Adult sizes and group sizes
For every species I'm considering, look up adult size and minimum group size from at least two sources. Take the larger figure when sources disagree. Add up the total cm of adult fish length the plan implies — being honest, not optimistic.
Stage 2: Surface-area test
Calculate the tank's surface area (length × width for rectangular). Divide by the total fish length from Stage 1. Compare to the 15–25 cm² per cm rule of thumb. If you're below the threshold, the tank is over-stocked for oxygen exchange even before you consider waste.
Stage 3: Length-of-tank test
Identify the most active swimmer in your plan. Multiply its adult length by 6 (8 for active cyprinids). Is your tank that long, end to end? If not, drop that species.
Stage 4: Bioload check via the calculator
Run our tank volume calculator with realistic dimensions to get the actual water volume (the 90% figure, not the gross). For a community of small tropicals (cardinal-tetra-sized fish, 4 cm and under), I'll allow up to roughly one cm of fish per litre of real water. For medium fish (6–10 cm), I'll halve that. For large fish (12 cm+), it depends entirely on the species.
This is conservative. A heavily planted tank with strong filtration can carry more. A bare-bottom tank with a moderate filter carries less. The numbers above are a starting point, not a ceiling — adjust upward only if you can defend the adjustment with specifics about your filtration, plant biomass, or water-change schedule.
A worked example
Suppose you have a 100-litre planted community tank, dimensions 90 × 38 × 35 cm. Filter is rated 800 L/h. You change 25% of the water weekly. You're considering:
- 10 cardinal tetras (adult 3 cm, school of 10 = 30 cm of fish)
- 6 sterbai corys (adult 6 cm, school of 6 = 36 cm)
- 2 honey gouramis (adult 6 cm, pair = 12 cm)
- 1 male Apistogramma cacatuoides (adult 8 cm = 8 cm)
Total: 86 cm of fish.
Stage 2 check: Surface area = 90 × 38 = 3,420 cm². Per cm of fish = 3,420 / 86 = 39.8 cm² per cm. Comfortable on the surface-area test — well above the 25 cm² per cm threshold.
Stage 3 check: Most active swimmer is the cardinal tetra. 3 × 6 = 18 cm minimum tank length. Tank is 90 cm. Comfortably passed.
Stage 4 check: Real water volume ≈ 90 L (90% of 100). Total fish length 86 cm → ~0.96 cm per litre. Right at my conservative ceiling for small tropicals — but most of the fish here are small (cardinals, corys), filtration is adequate (8× turnover), and the tank is planted. This passes.
The territorial check (informal stage 5): One Apistogramma male in 90 cm of length is fine — they want about 30 cm of territory, and the rest of the tank is occupied by non-competing surface (gouramis) and bottom (corys) species. No conflicts expected.
This stocking plan is comfortable, sustainable, and gives the fish room to behave naturally. It's also a long way short of what the inches-per-gallon rule would have allowed (which would be closer to 26 inches / ~66 cm — implying you could double the cardinal school).
The species the rule fails worst on
Some fish are so badly served by the inches-per-gallon rule that I'd encourage everyone to commit the exceptions to memory:
- Goldfish. Adult fancy goldfish need 75 litres each, plus 40 litres per additional fish. Single-tail (comet, shubunkin) varieties are pond fish, not aquarium fish, and need outdoor space.
- Plecos. Common plecos reach 45 cm and need 400+ litres. Bristlenose (the small ones) max out at 12 cm and are a reasonable community choice.
- Oscars and other large cichlids. An adult oscar is 30 cm and needs 280 litres minimum, alone or with similarly robust tankmates.
- Bala sharks, redtail catfish, iridescent sharks. Sold as cute juveniles. Become 30–120 cm fish that need pond-sized tanks. These are not beginner fish; in many cases they're not aquarium fish at all.
- Common goldfish at fairs. See above. They need pond space — please don't keep them in a bowl.
Closing
The inches-per-gallon rule isn't useless — it's a sanity check, and the fact that most experienced aquarists' final stocking ends up landing somewhere in the same neighbourhood is not a coincidence. But it gets there by accident, by averaging over the species the rule was implicitly built around (small tropicals, moderate filtration, weekly water changes). The moment your situation deviates from those defaults, the rule fails — silently, and often badly.
The four-stage check above is what a working aquarist does in their head every time. With practice it becomes automatic. Until then, do it on paper. Better still, do it on our tank volume calculator, which at least gives you the right denominator to start from.
Stock conservatively. Your fish will live longer, your tank will be more stable, and you'll have a margin when something inevitably goes wrong.