A 10 gallon tank, which is roughly 40 litres, comfortably holds a single shoal of six to eight neon tetras and nothing else. Neons are shoaling fish that need to be in a group to feel secure, and six is the sensible minimum. Eight is better still. That group fills the tank on its own, so resist the urge to add a second species alongside them.
Why the number is lower than the shop suggests
Neon tetras are tiny, so it is tempting to picture a dozen or more darting about. Two things hold the figure down. The first is the old inch-per-gallon rule, which counts only length and ignores that every fish adds waste your filter has to process. The second is that a 10 gallon tank is genuinely small, with little water to buffer a missed water change and not much swimming room once you account for plants and hardscape.
Six to eight neons give you a proper shoal that shoals, colours up and behaves naturally, while leaving the filter and the water chemistry some headroom. Pack in fifteen and you get a tank that needs constant maintenance and fish that are stressed rather than settled.
A few honest caveats
Neons want soft to moderately hard, slightly acidic water, so on very hard alkaline tap like mine they are not the easiest choice; a livebearer or a small hard-water fish suits that water better. Cycle the tank fully before any fish go in, because a small volume spikes ammonia fast. And keep them as the only fish. A 10 gallon is a single-species tank, not a community, whatever the shop stocking list implies.
If you want more going on in a 10 gallon
The honest answer of “one shoal, nothing else” disappoints anyone who wanted a busy little community, so here is the better move: pick the species to suit the tank rather than cramming more in. If your water is hard and alkaline like mine, a small group of endlers or guppies gives you constant colour and movement and actually thrives in it, where neons only tolerate it. If you have soft water and your heart is set on neons, keep the shoal and add nothing but a clean-up crew: a few nerite snails or a small colony of cherry shrimp share the space without loading the filter the way a second fish would. What does not work is a second nervy shoaling species or bottom-dwellers like corydoras, which need more floor and bigger groups than 40 litres allows. A 10 gallon rewards restraint, not a stocking list.
A quick FAQ
Can I keep neon tetras with a betta in a 10 gallon? It is tight and often stressful for both. In 40 litres I would keep one or the other, not both.
Is six neons too few? No, six is the minimum for a settled shoal. Fewer than that and they turn nervy and fade.
How many neon tetras in a 20 gallon tank? A 20 gallon (around 75 litres) holds a larger shoal of ten to twelve, with room to add a second small species.
To plan any tank properly rather than by rule of thumb, work from the true volume and adult size in the guide to how many fish you can keep, and run the numbers through the stocking calculator.