As a fishkeeper for 55 years I can tell you that a pool here will turn green unless it is partially or completely covered with roof. That is unless it has a big filtration system. I keep a community pool of tropical fish and aquariums. Decided to get few koi again this year knowing that cleaning the pool would increase tenfold. Nishikigoi (brocaded carp) and goldfish "poop a lot". I have minimal filtration but water straight from a waterfall source. So I just empty and replace some water now and then. Make sure you water is not chlorinated. If it is you will have to dechlorinate it.