There are several issues in this thread, which I will try to sort out as best I can. Some are very serious to the fish.
First to clear up about Prime, there is no need to use more than what will handle the replacement water. Seachem is a fairly reliable firm, but nevertheless they are in business to make money and getting us to use more of their products than we need will increase their business. But more to the point, all this stuff going in the water does affect fish, as the more conditioner you use the more chemicals and TDS are going in the water, and these do affect fish in varying degrees. The less the better.
To the initial question of DJbootleg. I have no idea of your level of experience or knowledge, but understanding how the many factors there are that affect the biology in an aquarium is key to success. As others have asked, we need to know the specific fish species and numbers in these tanks where fish are regularly dying. Are there any live plants in these tanks? When did the deaths begin, and can you relate this to any changes or new fish? What are their symptoms before they die? Do you see any aggression between fish, minor though it may be?
Until we know the above, there is little we can do to pinpoint the issue. But a couple of comments can be made about some of the factors mentioned already. Temperature is critical. All fish have a preferred range within which they will function at their best and thus be healthier. It is usually best to avoid the high end of these ranges long-term. As someone mentioned, higher temperatures can cause fish to become stressed, sick, and die, due to the increase in their metabolism this causes. Unlike us, fish are governed by their surrounding temperature and cannot regulate their internal temperature differently. A higher temp will not mean less infections, quite the opposite. It is true that raising the temp to 90F will kill ich, but you can't maintain this temperature for any fish. And ich is only one of a myriad of pathogens and protozoan that can affect and kill fish.
On the ammonia in the tap water, this is serious at a level of 2ppm. A water conditioner that detoxifies ammonia is essential (StressCoat does not), and in this case smaller and more frequent rather than larger and less frequent water changes may be needed, depending upon how fast the bacteria can take up the ammonia. Live plants will significantly improve this, as they are hungry for ammonia/ammonium as their preferred source of nitrogen. Water conditioners that detoxify ammonia only remain effective for 24-48 hours, and the pH is another factor.