I have 12 tanks, all with blue crayfish in them. 2 are 55 gallons, and I have 4 of each in them with no trouble. One of the 55's I put feeder fish in with them, and some do get eaten. In the other one, I have top swimming fish, molly's, swordtails, etc, and have had no problems. Some will be eaten in time, but I believe if you do it right, the losses can be minimal. Another super fish I've found to put with crays is tiger barbs. For just one cray, a 10 gallon tank would be fine. But I wouldn't have fish with any crayfish in a tank smaller than 29 or so gallons. The bigger the better if you are going to try to co-habitate.