Companion fir a male betta fish.

[AinaWGSD]

It really depends on his personality. Most betta fish actually prefer to be alone. The easiest tank mates for bettas are snails, but some of the more chill specimens can get along with other fish. You'll want to avoid anything nippy (barbs, for example) as well as anything with similarly long flowy tails (guppies, long tailed danios, etc). Some bettas can live companionable with shrimp, others will try to eat them.

The biggest problem is usually tank size because most of the fish that would make suitable tank mates are schooling fish and really should be kept in groups of 6 or (preferably) more.

Personally I think 2.5 gallons is too small for any fish. Just because they can survive in a stagnant puddle in the wild doesn't mean it's an ideal environment and most tiny betta tanks are really only intended for short term display. Minimum I would go is 5, my own betta is in a 10 gallon. The larger the tank, the easier it is to maintain water quality regardless of how many or few fish are in it.

 

[Rose the Legbar]

It really depends on his personality. Most betta fish actually prefer to be alone. The easiest tank mates for bettas are snails, but some of the more chill specimens can get along with other fish. You'll want to avoid anything nippy (barbs, for example) as well as anything with similarly long flowy tails (guppies, long tailed danios, etc). Some bettas can live companionable with shrimp, others will try to eat them.

The biggest problem is usually tank size because most of the fish that would make suitable tank mates are schooling fish and really should be kept in groups of 6 or (preferably) more.

Personally I think 2.5 gallons is too small for any fish. Just because they can survive in a stagnant puddle in the wild doesn't mean it's an ideal environment and most tiny betta tanks are really only intended for short term display. Minimum I would go is 5, my own betta is in a 10 gallon. The larger the tank, the easier it is to maintain water quality regardless of how many or few fish are in it.

I have always had my bettas in a 10 gallon and they thrive. The tank perimeters have always been good and the plants are also a great help. My current boy got downgraded to a 2 gallon temporarily (he has some fin rot and I am treating it) and he doesn’t seem pleased with such a small tank.
I think regardless of getting any tankmates, a 10 gallon would be a good upgrade for your boy and he should be much happier. If you want to test your bettas personality, you can buy a neocaridina and see if he chases it and bites it. If he does it won’t be the best of ideas to add any fish tankmates.

 

[Ilovemychicks08]

I had a male betta that we had put in with our guppies he did good for about 2 months and then went aggressive, so i pulled him out and put him in a 5 gal tank by himself.
Then added some shrimp and snails he did good with them.

He died at a few years old, but he didnt seem to mind not having other fish.