i kept f/w crays for about 2 years, they will eat your fish if they get a chance, they wait untill they are swimming past & grab hold of their tails, also snails are on the menu, we used to catch them at taupo round the boats using a piece of meat tied to a string & dangle it down to them, they would grab hold & you could slowly pull the string up & grab them. you need a lid on your tank because they climb up the airline, found this out the yukky way, got up one nite to use the loo & stood on something abit squishy...it had got out of the tank & crawled into my bedroom....