Stocking a 20-gallon tank

Why 20 gallons is a sweet spot — and a trap

Twenty gallons is large enough for a small tropical community and small enough that overstocking shows up as algae, nitrite spikes, and stressed fish. Stores often display mixed juveniles in tanks this size. Those fish are not at adult size, and the display is not a stocking recipe.

Throw away inch-per-gallon. Use adult size, schooling minimums, temperament, and your willingness to do water changes. When in doubt, stock less.

Example communities that fit

Peaceful school: 8–12 neon tetras with 6 peppered cories on sand. That uses the midwater and the bottom without stacking predators. A 20-gallon long gives better swimming length than a tall 20.

Livebearer tank: a small group of male guppies (skip females unless you want fry) with plants and perhaps a snail. Mixed sexes will balloon the bioload.

Betta centerpiece: one male betta in a planted 20-gallon, optionally with shrimp or snails if the betta ignores them. Adding a tetra school is possible only if the betta is not a hunter and you watch the first week.

What does not fit

Oscars, common plecos, full-size silver dollars, and yellow tangs (saltwater, and far too active) do not belong here. Goldfish, especially commons, outgrow and out-pollute a 20. Angelfish want height and become territorial; a pair is a squeeze in a standard 20-long and a poor match in a 20-high if you also want a community.

One of each species is usually worse than a proper school of one species. Six different 'centerpiece' fish will fight or hide.

Stocking order

Cycle first. Add the school or the bottom dwellers, wait, test, then add more only if ammonia and nitrite stay at zero. Leave headroom for the fish to grow and for you to have a bad week of maintenance without a crash.

Saltwater 20-gallons can hold a pair of ocellaris clownfish or a single firefish — not a tang. Marine nano tanks still need cycling, salinity, and a lid. See the species pages before you buy.