How the aquarium simulation works

One function runs the tank

Everything that changes over time goes through a single step that moves the tank forward up to 5 simulated minutes at a time. The same step runs while you watch, when you fast-forward, and when you come back after time away, so a week of catch-up ends in the same place as a week of live play. Catch-up stops at 7 days, so a long holiday doesn’t wipe out a tank in one go.

The nitrogen cycle

Fish give off ammonia as they breathe and digest, and leftover food rots into more. Two bacteria colonies live in the filter. The first eats ammonia and makes nitrite; the second eats nitrite and makes nitrate. By weight, each step makes more than it eats (2.7× and 1.35×), which is why nitrite can read higher than the ammonia it came from.

Each colony grows when it has food and slowly dies back when it doesn’t, and how fast it eats slows down as its food runs short. So bacteria settle at the size the tank’s waste supports. Add a lot of fish at once and ammonia shows on the test kit for a day or two while they catch up. Stop feeding a fishless tank and the colony shrinks.

Only nitrate stays behind. Plants take some up, and water changes remove the rest: every 30% of water you change takes away 30% of what’s dissolved in it.

Where the game is faster than a real tank

In the game, a new tank dosed with ammonia once a day cycles in about 5 days (we ran the model to get this number when this page was built). A real tank usually takes four to eight weeks.

The difference is on purpose. A new tank in the game starts with a tenth of the bacteria a cycled tank needs, as if they came in on the filter media, and a well-fed colony doubles in about 11 hours, at the fast end of what real nitrifying bacteria manage. No tank cycles in under 3 days. Weeks of waiting would teach the same lesson more slowly, so treat the game’s timings as a sped-up version of the real thing, not a schedule for your own tank.

Oxygen

Water holds less oxygen the warmer it is, and the game uses the standard curve for fresh water. The filter’s ripple tops it up at the surface, and an air pump adds more. Fish use oxygen in proportion to the waste they make, the bacteria use it as they work, and rotting food uses a lot. Plants add oxygen while the lights are on and use a little at night. In a badly overstocked tank, oxygen is what runs out first.

Fish health: symptoms before death

Each fish feels several kinds of stress: ammonia, nitrite, high nitrate, low oxygen, the wrong temperature, crowding, and too few of its own kind. Mild stress shows as a symptom you can see, like gasping at the surface or clamped fins. Only stronger stress does harm, and harm builds slowly: a fish in bad water takes at least 36 hours to die, and it shows symptoms the whole time. That gap is a rule the game is tested against, because a fish that dies without warning teaches nothing.

Hunger works the same way. An empty stomach first makes a fish thin, then weak, and a weak fish takes everything else harder. Fish also differ in how tough they are, so trouble doesn’t kill a whole tank in the same hour.

How full a tank is

The game and our compatibility checker share one crowding model: swimming space in each zone of the tank, plus total waste, where bigger fish count for far more than small ones. It is calibrated against published stocking lists. How the crowding model works. From the checker, “See it play out” opens a practice tank with your stocking and runs it a day a minute.

Facts versus model settings

The fish themselves come from this site’s checked species data: size, temperature and pH ranges, minimum tank and group size. The rest, such as how fast bacteria grow, how much waste a fish makes or how quickly stress does harm, are model settings tuned to behave like a real tank. They are not measured care facts, and we don’t present them as advice. For that, use the species guides.