I had an eletric blue crayfish and I never had an issue with it eating fish that occupied the tank with it. I did have another one about 11 yrs ago and I had to take him back to the LFS because at night it would hang from a plant in the back corner motionless until a fish got close and he would nab it. So when I got the 2nd one I remembered this and only had short plastic plants in the tank which seemed to help even if he climbed the heater or the corners of the tank.