Aquarium stocking calculator

How the aquarium stocking calculator works

The aquarium stocking calculator gives each species in its library an adult length, a bioload factor, a minimum tank size and a temperature range. The bioload factor stands in for how much waste a fish adds relative to a small tetra, so a messy four inch fish counts for far more than its length alone suggests. As you add species and quantities, the tool sums the total bioload for your planned community.

Capacity comes from your water volume, then adjusted by filter turnover. A filter that moves the water around five times per hour supports the baseline capacity, while stronger or weaker flow nudges it up or down within sensible limits. Percent stocked is the total bioload divided by that adjusted capacity. Below about seventy percent reads as understocked, seventy to one hundred as well stocked, and above one hundred as overstocked.

As an example, ten neon tetras in a 29 gallon tank with a filter rated for the tank comes out lightly stocked, leaving room for a small group of corydoras or a centerpiece fish. Swap those tetras for a pair of large mollies and an angelfish and the same tank climbs much closer to full, because body mass and waste, not fish count, drive the number.

Why the inch per gallon rule falls short

The old inch per gallon rule says you can keep one inch of fish for every gallon of water. It is simple, and it is wrong often enough to be risky. The rule treats a slim one inch tetra and a chunky one inch slice of a goldfish as equal, when the goldfish produces far more waste and needs far more room. It also ignores swimming space, schooling needs and the shape of the tank.

Waste output rises with body mass, not length, so a single large fish can pollute a tank faster than a dozen tiny ones of the same total length. This calculator uses adult size and a bioload factor for each species instead, which tracks real waste and space needs far more closely than a single inch based number ever can.

Bioload, filtration and oxygen

Bioload is the total waste your livestock produce, mostly ammonia from gills and leftover food. Your filter houses the bacteria that turn that ammonia into less harmful nitrate, so more filtration supports a higher safe stocking level. That is why the calculator raises capacity when your filter turns the water over more times per hour and lowers it when flow is weak.

Oxygen matters too. Gas exchange happens at the water surface, so a wide, shallow tank supports more fish than a tall, narrow one of the same volume, even though both hold the same gallons. If you plan to stock near the top of the range, add surface movement with the filter outflow or an air stone, and keep up with water changes so nitrate does not climb.

Reading the warnings

Percent stocked only covers waste load and rough space. It cannot see whether a species will outgrow your footprint, so the tool checks every species against its minimum tank size and flags any that need more water than you have. A school of active swimmers can sit under the stocking limit yet still be cramped in a short tank, which is why the minimum size warning is separate from the percentage.

Treat a warning as a prompt to rethink the list rather than a hard block. Sometimes the fix is a bigger tank, sometimes fewer of a large species, and sometimes a different centerpiece fish that suits your footprint. Behavior, temperament and water parameters still need your own research before you buy.

How to stock a tank safely

Cycle the tank before any fish go in. A new filter has no bacteria to process waste, so adding a full stock list to an uncycled tank is the fastest way to lose fish. Let the nitrogen cycle establish over a few weeks, then add livestock a few at a time so the bacteria can keep pace with the rising bioload.

Plan for adult sizes, not the small fish in the store bag, since most species grow. Keep schooling fish such as tetras, rasboras and corydoras in groups of at least six so they feel secure and behave naturally. Research compatibility before mixing species, because temperament, fin nipping and territorial behavior are not captured by a stocking percentage. When in doubt, stock a little lighter than the limit, since a lightly stocked tank is more stable and more forgiving of mistakes.

Signs a tank is overstocked

An overstocked tank tells you in several ways. Nitrate climbs quickly between water changes, algae spreads despite your efforts, and fish may hang near the surface gasping when oxygen runs low, especially in warm weather. Aggression and fin nipping often rise as fish compete for space, and disease spreads faster in crowded, stressed groups.

If you see these signs, reduce the load or increase support: rehome some fish, add filtration and surface movement, feed a little less, and change water more often. Prevention is easier than a cure, which is why it pays to plan the stock list with a calculator before you buy rather than after.

Stocking ideas by tank size

These are rough starting points for peaceful freshwater communities. Always confirm with the calculator for your exact volume and filter, and research each species first.

• 5 gallon: a single betta, or a small group of shrimp with a snail.
• 10 gallon: a small school of ember tetras or a few guppies, plus a snail.
• 20 gallon: a school of tetras with a small group of corydoras.
• 29 gallon: two small schools plus a centerpiece such as a dwarf gourami.
• 40 gallon breeder: a larger community with a bristlenose pleco and several schools.
• 55 gallon: room for a bigger centerpiece like angelfish with matching tankmates.