The short answer
In a 20 litre (about 5 gallon) tank, keep just 3–4 guppies, and ideally all male. Guppies are small but surprisingly messy, and a mixed group breeds so fast it will overwhelm a nano within months. A few males give you all the colour and movement without the fry explosion. Add a couple of shrimp or a snail for cleanup and stock nothing else.
Why so few?
The old “inch of fish per gallon” rule would suggest more, but it ignores bioload, adult size, temperament and filtration. Guppies produce a fair amount of waste for their size, and in only 20 litres the water chemistry swings quickly. Females also carry and drop batches of live fry every few weeks, so a “few guppies” can become dozens fast. Understocking is the safe, sane choice here.
Stocking guppies well in 20 litres
- 3–4 male guppies (or a small group of endlers, which stay even smaller)
- Add a cherry shrimp cleanup crew or a nerite snail
- Skip a second fish species — 20 litres has no room for a tankmate school
- Feed lightly; guppies beg constantly but overfeeding fouls a nano fast
If you want females or a breeding project, step up to a bigger tank so the fry have somewhere to go.
Before you add anything
Always cycle the tank first so the filter can handle guppy waste, then add them gradually and keep up small weekly water changes. For more room and mixed-sex options, see how many fish in a 50 litre tank, and for the tank itself browse the best nano aquariums.