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How many fish can I put in my tank?

The inch-per-gallon rule is wrong. What really limits stocking, worked examples from 30 to 200 litres, and how to add fish without a second cycle.

The rule everyone quotes, and why it fails

“One inch of fish per gallon.” It is the first answer any search gives you, and it is wrong in a way that matters. Fish are not lines; they are bodies. A fish twice as long is roughly eight times the mass, eats eight times the food and produces eight times the waste. By the rule, a 30-litre tank holds one 20 cm fish or ten 2 cm fish, and only one of those tanks survives the month.

The rule gets one thing right: volume limits load. What it misses is everything that decides how much load, and that is the list below. Work through it for your tank and the number falls out on its own.

What actually limits stocking

1. Waste versus removal. Fish produce ammonia; the filter turns it into nitrate; water changes take the nitrate out. A tank is overstocked the moment it makes nitrate faster than your changes remove it. This is the real ceiling, and it is measurable: if nitrate climbs 20 ppm or more between weekly tests despite a weekly 25 % change, the tank has more mouths than it can carry. Nothing else on this list overrides that number.

2. Adult size, not shop size. Every fish in the shop is a baby. The 4 cm “algae eater” is a common pleco that reaches 40 cm; the 3 cm angelfish becomes a 15 cm disc that needs 45 cm of water height; the 5 cm goldfish is a 25 cm fish that lives twenty years. Stock for the adult, and look up the species name, not the label on the tank.

3. Swimming room, and the shape of it. A 60-litre tank that is 60 cm long holds more fish than a 60-litre cube, because fish swim sideways and gas exchange happens at the surface. Active swimmers, danios and rainbows and larger tetras, need length more than litres; bottom dwellers need floor; a betta needs neither and mostly wants to be left alone.

4. Temperament and territory. One dwarf cichlid claims a cave and the 30 cm around it. Two male bettas claim the whole tank and each other. A tank’s capacity for peaceful schooling fish is far higher than its capacity for anything that holds ground.

5. Groups are not optional. Schooling species, most tetras, rasboras, danios, corydoras, are stressed and short-lived in twos and threes. Six is a minimum, ten is a school. This is the number that surprises people: a 60-litre tank comfortably holds a dozen small fish, but it holds them as two species, not six.

Worked examples

Freshwater, tropical, planted or not, with a filter sized for the tank and a weekly 25 % water change. These are conservative on purpose; a tank that runs under capacity has room for the mistake everyone makes.

Tropical freshwater, filter sized for the tank, a weekly 25 % change. One or two schools, a bottom crew, at most one centrepiece.

Under 30 litres is a one-fish tank, or an invertebrate tank: a betta, or a colony of cherry shrimp and a few snails. Above 200 litres the question changes from “how many” to “which”, because at that size the limits are temperament and adult length, and you can start thinking about a pair of angelfish or a single large centrepiece fish.

The pattern in every example: one or two schools of small fish, one group for the bottom, at most one centrepiece. Add species and you have to subtract numbers; the tank does not care how many kinds there are, only how much they weigh and how much they fight.

In the app — Stocking ideas runs this list for your actual tank: volume, heated or not, the latest readings, and everyone who already lives there. It suggests residents that fit, with proper school sizes, and flags what to watch out for with the ones you have. It is free, and it is the right place to test “could I add one more” before the shop does it for you.

Ask Finley what fits →

How to tell you are already overstocked

The tank tells you, in this order:

  • Nitrate climbs faster than the schedule. Twenty ppm or more between weekly tests, with weekly changes, is the definition. Bigger or more frequent changes hide it for a while; they do not fix it.
  • Ammonia or nitrite blips after every feed or every few weeks, in a tank that has been cycled for months. The bacteria are keeping up on average and losing on peaks.
  • Fish at the surface at dawn. Oxygen is lowest in the early morning, and an overstocked tank runs out first. If they are gasping at 7 a.m. and fine by noon, that is why.
  • Algae you cannot stay ahead of, because nitrate and phosphate never get low enough.
  • Chronic nipping and hiding. Fish with no room to get away from each other take it out on fins.

Fixing it is the boring answer: rehome, stop adding, feed less, and change more water until the numbers hold. A bigger filter helps with the blips, not with the nitrate; only water changes and fewer fish move nitrate.

In the app — every test lands on the nitrate trend, and the schedule advisor reads the slope: when nitrate has climbed 20 ppm across your recent tests, it says so with the numbers and proposes a tighter change cadence. If you keep having to accept that offer, the tank is telling you it is full.

Log this week’s test →

Adding fish without starting a second cycle

Your bacteria are exactly as numerous as the food they have been getting. Double the fish on a Saturday and the colony needs a week or two to catch up; in the meantime ammonia rises, in a tank full of fish. This is the single most common way a properly cycled tank still loses its first additions.

  • A few at a time. One school, or a group of six, per trip. Then two weeks before the next.
  • Test two days after and a week after each addition. A small ammonia or nitrite reading after new fish is the colony catching up; seeing it early is a water change instead of a loss.
  • Quarantine if you possibly can. Three to four weeks in a bare tank with a sponge filter. A shop fish has shared water with hundreds of others that week; ich, velvet and worse arrive in the bag. The guides on this site about ich and fin rot both begin with a fish that skipped this.
  • Acclimate slowly, and never pour the shop’s water into the tank.

In the app — add the new arrivals to the tank’s inhabitants and the schedule advisor proposes a test every two days for the fortnight the colony needs to grow into them; once three clean tests are in, it offers to ease back to weekly. Both are one tap to apply. The journal keeps the date, so a blip three days later has an obvious cause.

Log the new arrivals →

The number you were looking for, then: for a 60-litre tank, about a dozen small fish in two groups plus a bottom crew; for 100 litres, twenty in three; for 200, forty in four with a centrepiece. And whatever the number, the test kit has the final say.

Common questions

How many fish can I put in a 10 gallon (40 litre) tank?

One betta with a few shrimp, or a single school of eight to ten nano fish such as chili rasboras or ember tetras, or six pygmy corydoras. Not a betta and a school together, and not neon tetras, which want more length. Forty litres is a one-species tank.

Is there a better rule than an inch per gallon?

Not a single number. The honest rule is the nitrate test: a tank whose nitrate climbs less than 10 ppm a week on a weekly 25 % change has room, and one that climbs 20 ppm or more is full. Everything else on this page is about predicting that result before you buy the fish.

How many fish can I add at once?

One school, or a group of six, then two weeks before the next. The filter’s bacteria grow into the extra load over about a fortnight, and a tank stocked all at once gets a second cycle with fish in it. Test two days after each addition and again a week later.

Does a bigger filter let me keep more fish?

It helps with ammonia and nitrite peaks, which is worth having. It does nothing for nitrate, because filters produce nitrate rather than removing it; only water changes and fewer fish move that number. A bigger filter on an overstocked tank is a tank that is overstocked more reliably.