Guide
How many fish for a 120 litre tank?
By the Aquarium Calculator team
A 120-litre (about 32 US gallon) aquarium comfortably holds around 25–35 small community fish, such as a school of tetras, a group of corydoras and a few centrepiece fish, provided it's cycled, well filtered and given weekly water changes. The exact number depends on the species' adult size and waste, not a simple fish-per-litre count.
Start with the real water volume
A tank sold as 120 litres usually holds less water than that. Typical 120-litre tanks measure around 80 × 35 × 45 cm on the outside; after subtracting the glass, the gap below the rim, a substrate bed and some rock or wood, the real water volume is often closer to 95–105 litres. Stocking, filtration and dosing should all be based on that net figure, which you can calculate for your exact tank with the volume calculator.
Shape matters too. A 120-litre tank that is 100 cm long gives schooling fish far more swimming room and surface area than a tall 60 cm cube of similar volume. Fish live in the horizontal plane, so a longer footprint supports a little more stock.
Why the "1 cm of fish per litre" rule fails
The common rule of thumb says one centimetre of adult fish per litre of water, which would suggest 120 cm of fish for this tank. It's a rough starting point at best. It treats a slender 3 cm neon tetra and a chunky 3 cm juvenile goldfish as identical, even though the goldfish will grow to 20 cm or more and produce many times the waste. It ignores swimming space, territorial behaviour and schooling needs entirely.
A better approach is bioload: how much waste each species produces relative to its adult body mass. Small, slim fish like tetras and rasboras have a tiny bioload; heavy-bodied fish like goldfish and plecos have a large one. The stocking calculator assigns each species a bioload factor and measures the total against your tank's net volume and footprint, then tells you how much filtration and how many water changes your plan needs.
Three stocking plans for a 120 litre tank
Peaceful community: a school of 12–15 small tetras such as neon or ember tetras, a group of 6–8 corydoras for the sand, and a pair of honey gouramis or a single dwarf gourami as a centrepiece, with a few nerite snails for algae. This keeps the bioload moderate, gives every schooling species a proper group, and fills the top, middle and bottom of the tank.
Livebearer tank: 6–8 platies or a single-sex group of guppies, a school of 10 harlequin rasboras, and 6 corydoras. Livebearers breed readily, so plan what to do with fry, or keep males only. Nano planted tank: a large school of 20–25 chilli rasboras or celestial pearl danios with a colony of cherry shrimp and some otocinclus. Tiny fish in big groups look spectacular in a planted 120-litre tank and keep the bioload very light.
Fish to avoid in 120 litres
Some fish commonly sold to beginners outgrow a 120-litre tank. Common plecos reach 30–50 cm, oscars over 30 cm, and common or comet goldfish need far larger tanks or a pond. Fancy goldfish are possible but heavy on waste: a 120-litre tank suits about two fancy goldfish on their own, not a community. Large or aggressive cichlids, iridescent sharks and clown loaches (which grow large and need big groups) also need much bigger tanks.
Compatibility matters as much as numbers. Tiger barbs and some tetras nip the fins of slow, long-finned fish like bettas and fancy guppies, and every species in the tank must share an overlapping temperature and pH range. Check each fish's needs in the species directory before you buy.
Equipment and maintenance for a full 120 litre tank
For about 100 litres of real water, aim for a filter delivering 400–600 litres per hour of real flow, which usually means a filter rated for 150–200 litre tanks. A 100 W heater holds 25 °C in a typical heated room; use 150 W, or two smaller heaters, in a cold room. Plan on changing 25–30% of the water every week, about 25–30 litres, and test nitrate to fine-tune the schedule.
Above all, don't add everything at once. Cycle the tank first so the beneficial bacteria are established, then add fish in groups over several weeks, testing ammonia and nitrite after each addition. A gradually stocked, moderately stocked 120-litre tank is one of the most rewarding and stable setups you can keep.
Check your own tank
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