[Mergatroyd]

I kept a 20 gallon tank for years as a kid, always had a betta, never had trouble. Now I have a 10 gallon, and after a few months of peaceful harmony, the betta is now bullying the cories and keeps them crowded into one corner. I assume they feed at night, but during the day they all crowd into one corner, and betta circles and flares at them. I bought some guppies in hopes that when they breed th betta will amuse himself stalking the guppy fry and leave the poor catfish alone. Long term plan is to get a larger tank for the cories and other fish and let the betta have the 10 gallon to himself with some tetras for company (he leaves them alone) and some sort of bottom feeder for clean-up.

I think some bettas are way more aggressive than others.

 

[Mergatroyd]

I'm wondering how much tank size is a factor. Have kept a individual bettas in a communal 20 gallon tank in the past with not problem, but not having good luck in a 10 gallon now: betta is terrorizing cory cats.