Bill,
Could be my cages had ice build up etc. due to their size (at least 7 X 7) and I did have a diffuser running in the vicinity that first year.
I still feel safer dropping them down into a warmer layer, and I probably have a higher amount of fish than you do. About 350 five to seven inch bluegills right now in one of those 7 X 7 cages and 200 SMB in another 7 X 7 cage.
We'll see how they come out come spring.
