JeffHAP, It depends on the fish species. Generally for warm water fish no problem for not having a cool area or temperature differential. Remember we are talking about cold blooded animals. Generally the only time the lack of a cool water refuge or temperature differential is with cold (trout) or cool water (yellow perch) fish. And more so with cold water fish than cool water fish.
For deep ponds (>25ft) that stratify and loose DO in the deep zone there are hypolimnetic aerators that can oxygenate the deep cold water zone/layer and not destratify the pond/lake.
