Just did an ordinary w/c, added Prime, nothing unusual. I sit here and see my fish are dying. At least 2 are gone. This has happened before, a while ago, and it turned out the water company was working on the water pipes. This time no clue. One of the fish is a big Rose-lined barb aka Denison's Barb. This has me so, so upset. What can I do in the future to prevent this when I have no clue as to what is wrong? What should I look for?