On the other hand, there are a number of fish that we keep in warm waters that can stand, and prefer, cooler water. Cory's come to mind. While not cold weather fish by any stretch, there are some that prefer cooler water than what we keep them in. Also, I believe that the biggest issue is in temp changes. You might not be able to put your fish out until say June, but then they get conditioned to the temperature and with a large enough tub the temps do not swing that drastically. Having said that, there are always exceptions. Here in the Northeast we just had snow in mid May. Unusual but not unheard of. So, give it a shot, keep an eye on the placement of your pond (shade/sun) and the temperature in it, both mid day and say first thing in the A.M.