How Many Fish for Your Tank Size

Written 2026-09-30

Tank volume alone doesn’t tell you how many fish it can hold. We ran the Water Kiwoogi simulator with the same filter (a large sponge filter) across three tank sizes and just kept raising the neon tetra count: a 45cm tank (about 36L) topped out at 15 fish, a 60cm tank (about 65L) at 26, and a 90cm tank (about 182L) at 44 — all with zero deaths over 90 days. Volume grew 5x but safe stocking only grew 2.9x: if you size up the tank without sizing up the filter, stocking doesn’t scale with volume. Swap the 90cm tank’s filter for a large canister filter instead, and it held 64 fish with no deaths at all over 90 days. Swimming space is a separate gate, too — zebra danios, which swim in fast straight lines, all died in a 45cm tank even though 5 fish is a tiny bioload, while the same 5 fish lived the full 90 days in a 60cm tank.

Volume isn’t enough — keep the same filter and stocking won’t scale

It’s tempting to assume a bigger tank dilutes waste more, so stocking should scale with volume. We set up 45cm (45×27×30cm, about 36L), 60cm (60×30×36cm, about 65L) and 90cm (90×45×45cm, about 182L) tanks, each with just one large sponge filter, let them finish cycling, then added neon tetras all at once on day 30 and watched for 90 days. One extra fish was enough to tip each tank into deaths: at 16 fish the 45cm tank lost 2 starting on day 89 (nitrite peaked at 2.2ppm on a hobby test kit), and at 28 fish the 60cm tank lost 6 starting on day 78 (nitrite peaked at 2.62ppm). Going from 45cm to 60cm, volume grew 1.8x and safe stocking grew 1.7x — roughly proportional. But going from 60cm to 90cm, volume grew 2.8x while safe stocking grew only 1.7x. When the filter stays the same, a bigger tank only takes as much as that filter can actually process.

Swapping the 90cm tank’s filter changed the picture. With the sponge filter, 44 fish was close to the ceiling. With a large canister filter instead, 64 fish lived the whole 90 days with nitrite never rising above 1.37ppm on a hobby test kit. Sizing up the filter along with the tank is what actually lets stocking grow.

Swimming space is a separate gate from volume

Zebra danios swim in fast, straight lines without stopping, so the simulator requires a tank at least 60cm long for them. We put just 5 zebra danios in a 45cm tank (45cm long, about 36L) and every one of them died, starting on day 72. Ammonia and nitrite never rose at all during those 90 days — this wasn’t a water quality problem, it was a lack of room to swim. The same 5 fish in a 60cm tank (60cm long) all lived the full 90 days with zero deaths. Clean water and plenty of volume don’t help if the tank isn’t long enough for how that species actually swims.

Instead of a rule of thumb like "one inch per gallon"

Rules of thumb that boil stocking down to a single number flatten out conditions that really differ by species, so they’re sometimes close and sometimes way off. The simulator tracks at least three separate things: ① how much waste the filter can actually process (the filter’s capacity, not the tank’s volume, sets the ceiling), ② the minimum group size for schooling species (too few and the stress itself builds up), and ③ the swimming length active species need, which has nothing to do with depth or volume. Check the species catalog for the fish you want and the tank size you’re considering, then try the real numbers yourself with the button below.

The button below is the 60cm tank · large sponge filter · 26 neon tetras setup from this article.

The numbers in this guide come from the Water Kiwoogi simulator model; real tanks can differ.