I was waiting for someone to ask. A degree is the average water temp for the day. Therefore if the water is 6 degrees, that is a 6 degree day. So therefore you keep a record of the daily temp and accumulate it. Hence 1000 deg days at a water temp of 10 deg C equals 100 normal days. Hope that helps. Eggs and Alevins are kept at approx 6 degree and fry (pic #4)at approx 12 deg. Steve