Why Small Tanks Are Actually Harder

Written 2026-10-25

A nano tank looks like the easy, low-maintenance option, but running one through the Water Kiwoogi simulator shows the opposite. Two things work against it — ① it often holds far less water than its name suggests, so it falls short of what many species actually need, and ② the same mistake concentrates waste much faster and much higher when there’s less water to dilute it in. A "20 cm cube" nano tank only holds 6.8 L of actual water once you subtract headspace. Put a single betta in it and even with flawless water quality, it starts failing around day 43 and eventually dies — simply because the tank is smaller than the betta’s minimum size requirement (15 L). And if you make the classic beginner mistake of adding shrimp before the tank has finished cycling, the same two cherry shrimp die by day 11 in a 6.8 L nano cube, by day 22 in a 24.3 L cube, and only one dies — on day 49 — in a 59.4 L standard 60 cm tank. Less water means the same mistake hits sooner and harder.

It doesn’t even hold enough water — a betta can’t survive a 20 cm cube

The name "20 cm cube" suggests room for one or two small fish, but the real water volume is much smaller than it looks. Water height is measured from 3 cm below the rim, so a 20×20 cm floor at that height works out to just 6.8 L. Many of this simulator’s 72 species need a minimum tank size somewhere in the 15–40 L range, so 6.8 L falls short of almost all of them. We put a single betta (minimum 15 L) into a 20 cm cube with perfect water quality — ammonia and nitrite never rose above zero on a test-kit scale — and it still started losing health on day 43 and eventually died. The only cause logged was "tank too small," never temperature, pH, or water quality. That’s a loss you can’t manage your way out of. Only invertebrates with a 10 L minimum, like cherry shrimp or snails, just barely scrape by in 6.8 L — with no margin to spare.

The same mistake hits small tanks sooner and harder

In this simulator, ammonia concentration builds up as "waste produced per hour ÷ water volume." With less water, the same amount of waste raises the concentration much faster. To see this directly, we made the single most common beginner mistake — adding livestock before the tank has finished cycling — in three different tank sizes. We dropped two cherry shrimp into each tank on day one, with no bacteria starter, and watched for 90 days. The smaller the tank, the faster the deaths and the higher the nitrite spiked.

As water volume climbed from 6.8 L to 24.3 L to 59.4 L, the first death moved later and later — day 11, then day 22, then day 49 — and the nitrite peak dropped from 3.62 to 1.28 to 0.55 ppm. Same number of animals, same mistake, but more water bought more time and did less damage. A nano tank has almost no cushion for that, so an ordinary day-or-two delay in reacting turns fatal fast.

If you still want to keep a nano tank

None of this means a nano tank is impossible. We put the same two cherry shrimp into the same 30 cm cube (24.3 L), but this time waited for the tank to finish cycling first — 30 days with a bacteria starter before adding anything. Over the next 90 days, ammonia and nitrite never rose above zero on a test-kit scale, and both shrimp survived. Keeping a small tank really comes down to three things: ① pick only species that comfortably clear the tank’s minimum size (a true nano like the 20 cm cube leaves you almost no options at all), ② wait patiently until cycling is genuinely finished, and ③ stay stricter about water changes and feeding than you would with a bigger tank. The less water you have, the less room there is to recover from a mistake.

The button below is this guide’s last example — a 30 cm cube with 2 cherry shrimp, added only after cycling finished.

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