Pond Boss Forums POND CONSTRUCTION Creating habitat Water bugs

My little 200 gallon minnow and lily propagation pool has been cleaned out of minnows (thousands of rosies and then gambusia) twice, by what I presume are water beetles (like the toe biter). Forty minnows a day, their obviously punctured but whole bodies on the bottom, until there just ain't no more minnows after some weeks time. I've watched hellgrammites capture minnows, but they dissolve out the minnow until there's nothing but a layer of scales. Thus, I presume, killer beetles...they actually have a breathing tube and can hang under a lily pad. But I never actually saw one.