it sounds like the phosphates are coming from the tap, since Jeff said the problem is worst right after WC's. Yeah, a good idea then would be to test your tap water. If it's really bad, maybe an RO unit is in order? That would get really expensive for your large tanks, tho.
As for the problem being worst in the summer, I've heard that municipal water supplies will add all kinds of different things during certain times of year. Like in summer, there can be lower pH in your tap water. I don't know why, but maybe it facilitates treatment of water in different season/temps.
As for the problem being worst in the summer, I've heard that municipal water supplies will add all kinds of different things during certain times of year. Like in summer, there can be lower pH in your tap water. I don't know why, but maybe it facilitates treatment of water in different season/temps.