Stocking starts with volume, not with a species list

This is the part of the hobby where the internet is least useful, because the question people search - what fish can live with a betta - has an answer that depends almost entirely on a number they did not mention. So we are going to state the threshold before the list, every time, on every page in this hub.

At 5 gallons: invertebrates only. A nerite snail, or a couple of Amano shrimp, and even those are not guaranteed. No fish. Not a single corydoras, not one otocinclus, not three neon tetras.

At 10 gallons: fish tank mates become possible. Possible, not certain - a species that works in most tanks will still be killed in yours if your betta is one of the individuals that will not tolerate anything.

At 20 gallons: you have a community tank that contains a betta, which is a completely different and much easier proposition.

And the sentence that costs us money to write: a betta needs no tank mates at all. It is a solitary, territorial fish that does not shoal, does not pair-bond, and shows no sign of benefiting from company. Tank mates are something you want. That is a perfectly good reason to want them, but it should be an honest one.

Why volume is the whole argument

Three separate mechanisms, all of which scale with water:

Territory. A betta patrols. In 4.3 gallons of actual water there is nowhere for a second animal to be that is not inside the betta’s territory. At ten gallons there starts to be a far end.

Line of sight. Aggression in this species is largely triggered by seeing something. Volume buys you the ability to break sight lines with planting and hardscape, which is why a heavily planted ten-gallon works when a bare one does not.

Bioload. Every animal you add produces ammonia into a fixed volume of water. Doubling the stock in a 5-gallon is a far bigger change to water chemistry than doubling it in a 20, and the fish that dies of that is usually not the one you added.

What actually works, by volume

At five gallons, a single nerite snail is the safest addition there is: armoured, slow, and no threat to anybody. Amano shrimp work more often than they fail, but they are betta-sized snacks and you need dense cover for them.

At ten gallons and up, the species that work are the ones that are neither long-finned nor brightly colored - a betta attacks what looks like a rival betta - and that occupy a different part of the tank. Bottom-dwellers in a group, or a small shoal that stays mid-water and does not trail fins.

The species that reliably do not work, at any volume: male guppies (long fins, bright color - they read as rival bettas), any fin-nipper, and any second betta of either sex outside a properly managed setup.

The mistake buyers make

Adding the tank mate before the tank is cycled, and adding it to a tank that has no cover. Cover is not decoration. In a stocked betta tank it is the mechanism that makes the stocking work.