How many Bettas in 3 Gallons?

a three gallon is a perfect size tank for cherry shrimps.
 
One betta only in a 3g.

My advice would be to keep your 3g as an isolation tank if you have a sick fish that you have to separate, or to quarantine a new fish. It is really too small to be a permanent home for any fish. Bettas do love space, and when they have it they are not an aggressive fish. I have had three female bettas quarantined in an 8g. They went fine for 3 weeks, then one of the females decided that tank was her territory. I cut the quarantine time short and put all three in the 50g community tank, no more aggression.

I currently have a betta in a 12g with 10 cardinals. They get along great.
 
I have one betta in a filtered, heated, 2.5gal. He's fine in there, but I wouldn't ever think about putting more than one in it. Nor any neighbors for that matter.
 
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