So it starts off clear and then turns a yellowish pee color within 30 minutes? That's weird. That sounds like either some foreign liquid or matter is getting in there and discoloring the water, or it's some kind of chemical reaction. Are you sure nothing could be slowly trickling in after it's flushed with clean water? Maybe a roof leak over the toilet? I remember we used to have a trailer with persistent roof leaks after hail damage, and the water in the floor would often be that color. It didn't have to be raining because the water was already up over the ceiling panels and would drip down through the ceiling lights whenever it felt like it.