Last summer the water temp was perfect to do my weekly water changes straight from the hose. That's what I did every week. Suddenly, on Sept 2nd, in the middle of a water change all the fish started gasping at the surface. I lost a silver dollar pretty quickly. I doubled dosed the prime and hoped for the best, and by morning everything was ok. I looked for my water bill so I could call the water company and see what was up....sure enough right on the bill, "Chloramine treatment begins 1 Sept".
