Yeah that would tell me there is not enough D.O. down there and if they are all staying in one depth in winter they could be using a lot of D.O in that area so a winter air setup at least to keep that water flowing and making new D.O. would benifit you I think. Colder water holds more D.O. but if all or a lot of your fish are in the same depth water that could present a D.O. problem. Then if you did decide to use it in the summer you could put it in the 18 foot deep area and maybe get some of those fish to use that area too.
