As RockandRoll stated there are probably other factors that played a role in the death of your fish. I use tap water all the time with no problems, I only add Prime to it as the water conditioner. Have you tested your tap water to see what the ammonia, nitrites & nitrates are? Do you have filtered, heated tank? What size is it? How often do you do water changes & how big are they? Was he acclimated appropriately? How long did you have him before he died? And so on and so on. I'm not asking you answer the questions just pointing out there are numerous things that could have contributed to his demise. I think we've all been there at some point in time, it's a sad part of fish keeping.