Small aquariums are sold as the answer for people short of space and experience. In practice it is the other way round: the less water there is, the harder it is to keep it in order. This article is about what can really live in 10–20 litres, and what must never go in there.

Why a small volume is harder

Water is a reserve of stability. Everything that goes wrong in an aquarium goes wrong faster and harder in a small one:

  • Temperature. Twenty litres by a window lose several degrees overnight, and a sunny day overheats them. In a hundred-litre tank the same room changes almost nothing.
  • Chemistry. An uneaten pinch of food disappears almost without trace in a hundred litres; in fifteen it causes an ammonia spike. A feeding mistake that passes unnoticed in a big tank becomes poisoning here.
  • Oxygen is scarcer in a small volume too, and it leaves faster when the water warms.
  • There is no room. A shoaling fish needs length as well as water: even small species need somewhere to pick up speed.

Hence the rule: a small aquarium is not "the beginner's version" but an option for someone who can already hold parameters steady.

What the catalogue data says

We looked at how many of the 1249 species really fit into such a volume.

  • Up to 15 litres — one species in the whole catalogue.
  • Up to 20 litres — fifteen. Almost all of them are parosphromenus and related blackwater fish: demanding species that need soft acidic water, subdued light and experience. They cannot be recommended to a beginner.

For comparison: the Siamese fighting fish, most often sold together with a three-litre jar, is listed in the catalogue at 40 litres. That is not strictness on our part — it is what a fish that lives up to five years needs.

Who really fits

  • Neocaridina shrimp and snails. The honest answer for 10–20 litres is invertebrates rather than fish. The volume is enough for them, they are interesting to watch and they do not suffer from lack of space.
  • One or two dwarf fish species — those listed in the catalogue with a minimal volume, and only in a mature tank with regular testing.
  • A specialist species tank — parosphromenus and their kin. That is not "nano for beginners" but a pursuit of its own, for someone prepared to work at the water.

Who must not go in

  • Goldfish. They need hundreds of litres: large, long-lived and very messy.
  • Shoaling fish — neons, danios, rasboras, guppies. A group of six in fifteen litres lives cramped, and aggression within the shoal is common in such conditions.
  • Bottom-dwelling catfish, corydoras included. They need floor area and a group of their own kind.
  • Any fish "to grow into it". A juvenile promised 20 cm will not stay smaller because the tank is.

If the tank is already bought

  1. A heater with a thermostat — always, even if the room "is warm anyway". Night-time drops do more harm than a couple of extra degrees.
  2. A sponge filter on an air pump — it creates no strong current and does not draw in small inhabitants.
  3. A lid or cover glass: a small tank evaporates fast, and with the level the salt concentration changes too.
  4. A 20–30 % water change once a week, matched in temperature and parameters. A complete change is never acceptable.
  5. Ammonia and nitrite tests — in a small volume they are not a luxury but the only way to spot trouble in time.
  6. Plants — mosses, anubias, floaters: they take up some of the nitrogen and provide cover.
  7. Feed less than feels right. In a small volume overfeeding is the commonest cause of death.

An honest conclusion

If the choice has not been made yet, a 40–60 litre tank is simpler, cheaper to run and far more forgiving than a fifteen-litre nano. The difference in price is small; the difference in reliability is fundamental.

On starting any aquarium properly, see the article Starting an aquarium and the nitrogen cycle; on water parameters, the article Water chemistry.