OK, I would almost guarantee thats your problem. I was having issues last year with fish getting sick. But only a select few. I had my water company come out with their testing equipment and test the water. Straight from the outside faucet, the water was decent. From the sink faucet all the way cold, it was coming up with some metals in it. On warm, OMG :affraid: I could not believe the metal readings. Lead, copper, zinc, you name it, it was in there. So we tested that water after the addition of Prime. The readings were still crazy high. We then added Rid Metals to it. That worked well, but the readings were still to high for me to be comfortable. Then we tested warm water after treating with Prime and Rid Metals and letting it sit for 24 hours. Water was perfect. So, long story short. If you want to do water changes with 5 gallon buckets after treating and sitting, use warm water. Otherwise, use water straight from the outside faucet if possible. If not, straight cold water. The fish actually dont mind the mild drop in temp, it kinda simulates the rains.
Since this, I have had no issues.
: )