Advice please; Tank mate for Betta in specific 5.1g setup.

Perhaps an African Dwarf frog or two? I've also kept shrimp with bettas, and a couple of cory cats would likely be fine. A bristle nose pleco might be too large for that tank, but they get along with bettas. Be careful with them though, some plecos get 2 ft.

GK
 
Maybe food for thought:
Today I replaced the HOB filter in my Betta tank with an ATI #1 sponge filter.
What a difference.
He's no longer swimming around with his fins half folded.
Now he displays himself in all his glory.
Of course I had to wrap the air pump in a towel, put it in a box, and then stick it in the closet, so I didn't have to listen to it pulsing away.
Cheers...
 
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