I have an 20 foot round above ground pool (easy set frame type ) that I leave up all winter long here in zone 5. I have been doing this for five years now (first several years I took it down for the winter), I don't cover it for the winter for fear that the weight of the snow and ice might bring down the walls (not as strong as a "real" above ground pool). I close it once the water drops below 60 degrees or so and open it at the same temperature, thinking algae growth stops at those temps. I have not had a problem with this until this year. This year the water was green even before the ice had completely melted! Water temps are about 50ish and it is almost a green soupy mess!!! I think I am going to end up opening the pool and it is only March! I am not really worried about shocking and cleaning up the algae, my question is why the heck did this happen with water temps so low?