Stocking a 2.5g

Why not just buy a 5 gallon tank.

Really fish wise endlers are about it besides a betta. And even that really isn't great. Not apple snails but other snails will work. Freshwater shrimp would look cool.

2.5 gallon tanks are hard to keep stable so you need something hardy and adaptable. Bettas fit that bill, though they do best in heated tanks with a light filter and live plants.
 
Maybe a few dwarf puffers. I saw them at the lfs for the first time in "real life", and I thought they were the neatest things. Now the only problem I'm having is where am I gonna put another tank for them :D
 
You should really have 5 gallons per dwarf puffer. They may be small but are very messy fish and need a filter and heater.
 
You could probably put 2 or maybe 3 fancy guppys in a 2.5g. I had 2 in a tank that size for awhile and they seemed happy but they were kind of hard to see because they hang out at the top alot. Moved them to my 35g. I just bought a heater for a 2-5g tank on www.drsfostersmith.com
I had a betta in the 2.5g after the guppys, then I got a good deal on several tanks so mr. betta got upgraded to a 10g all to himself, but I might like another betta for one of the 2.5g tanks. They are really neat fish if you've never had one, this is my first and I am quite impressed with him!!
 
Maybe a trio of least kilifish. I'd not put guppies in anything smaller than a five, and even then I would only put a pair for serious breeding purposes.
 
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