A Betta splendens will definitely eat lots of fry if he is given the chance. I keep one in my big endler tank to help me hold the population explosion to slightly less than it would be without him. My other betta is in my 120 gallon community tank and seldom gets a chance to eat fry because he is not as fast as the platies, rasboras, barbs, angels and other fish in the tank. They always get the fry first.