Our pool, that we, people often swim in, swallow an amount of water, etc., once turned VERY green.
My parents took a sample to the pool place, and they said "do you have a lot of squirrels in your yard?"
"no..."
"well your pool is full of animal pee"
UGH. We had to teach Milo to pee before going in the pool and get out periodically to potty. Fortunately that seemed to work.