We really need more data here, we are just jumping around without. Test the tap water itself for ammonia, nitrite, nitrate and pH and post the numbers. For pH, you need to out-gas the CO2 from tap water before testing, so you can let a glass of water sit 24 hours, then test--this will be more accurate.
Also test the tank water for the same things, and post the numbers. In both cases, we need the actual numbers.
Do they add chloramine to your water, or just chlorine?
Do you have live plants? What is the fish load (species and numbers of each)? A photo of the tank would be helpful too.
As has been mentioned above, Prime and most similar products that "detoxify" ammonia do not remove it, they bind it (usually into ammonium which is harmless) and this lasts for 24-36 hours. "Ammonia" test kits will still show ammonium as "ammonia," even though it may be harmless at the time. Ammonium can revert back into ammonia. But if the pH is below 7, "ammonia" is primarily ammonium so not such a problem. But we still want to pin this down.