Electic blue crayfish

You will need to provide it's own tank. Doesn't need to be big. No heater. Good filtration. A hangon back like an aquaclear is good because they like the current. Crayfish will eat fish. They WILL catch them eventually, especially the betta and bottom feeders. I loved my electric blue crayfish, especially teh female, they get huge. My mother left the cover open when I went away to school and they escaped. You have to keep all openings covered.

what size tank do you need for one of those? Can you have more than one in a tank?

They fight. You can breed them but they fight a lot. You'd need at least a 55 gallon for two I'd think. I small tank is all you need for one. They don't move around that much. They are very active when young and become less so as adults.

As far as the aquarium is concerned, it would be ok. However, as far as the fish are concerned, you have quite a few bottom feeders, and some other slow moving fish, which could fall victim to it. Fast moving fish would do fine with it, I owned one a few years back and it ate my cories. They are however a long lived crustacean, and given proper conditions will live for quite a few years....I had mine for 5.