I already have comets (small ones), so they would need to get along. But not big enough so they could eat the comets (they're only 1-2" long). I would like to keep them out for the winter, but if they do need to come in i'll get a tank. The pond is 7' long x 5' wide x 2-2 1/2' deep. I live in Cincinnati, OH, so the pond freezes on the surface, but I'll get one of those heaters to leave a hole in the surface.