For the last 6 months have been doing a 50% water change every two weeks in my 55g tank (with 25g sump for total of 80g). I use an overflow in the sump that lets excess water go to the drain. That way, I have no buckets to carry. I slowly trickle in tap water (20g per hour) that is at the same temperature as the tank water. I check this temp every 15 minutes to make sure it has not drifted. After the new water has been added, I treat with a chlorine remover and add fertilizers for the plants. Today I stopped the water change early because I saw that the fish were stressed. I added the chlorine treatment and fertilizer, and watched as 6 Cardinals died over the next few hours. I even added more water treatment, and it did no good. None of the other fish died. This tank is heavily planted, and has been running perfectly since it was refurbished 6 months ago. I am suspecting that my local water utility flushed the system with extra chlorine or something. Anyone else ever had this happen?