I own four betta fish. A four year old male who is doing just fine, and three females who share a decent sized tank. I have never used a filter with my male, so I figured that I wouldn't need one for the females. Am I right or wrong to do that? Some people have told me that even bettas need a filter, but I can't really say I agree because of my male. Now, I'm wondering if I could put any other fish in the same tank as the females. Depending on your opinion of the use of a filter, are there any fish I could put in? My females have hardly ever been agressive towards each other, so I assume they would be fine with a tankmate. My major question is are there any fish other than bettas that don't require a filter to survive and be healthy?