i've had a 20 gallon for about 4 months and just recently added a new blue lobster/ crayfish. the lady who sold it to me at my local aquarium store said there shouldnt be any problems with my other fish (a twin-tailed betta, small chinese algea eater, and white tipped shark cat) but even after 30 minutes, the lobster built his home underneath a rock and started harassing the betta and cat. i thought it was initially just trying to become familiar with its surroundings but soon realized that he ACTUALLY waited for the slower fish to swim by and came out and attacked them, grabbing on to the bettas lower tail. now the lobster is in a two gallon bowl awaiting whatever fate becomes him, probably returning him back to the store, but i was wondering if there was anything else i could do.