Just above freezing, but try keep the low above 40F.
I'm going to try keep things in the upper 60's, so I'm setting my low (for media volume) in the upper 50's.
I only have specific engineering and performance data on one particular brand of media, so I can't comment on others that most would be familiar with. With that said, at 59F feeding 1lb of 40% feed/day it requires 1.3 cubic feet of media. If it drops down to 40F, the requirements bump up to 1.8 cubic feet. Best to toss a 25% buffer on top of that.
