Fish keep dying!!!

A few weeks ago I had picked up some new fish from the LFS (box store), and when I put the fish in they died within about 5 minutes. I couldn't figure out what the problem was as my parameters were all fine. I just guessed that it was due to the difference in Ph as the LFS kept there Ph very low. Well I picked up some new F1 fish last week and changed my tank all around. Today I noticed a couple of them had a few white spots on them, so I dosed with some Super Ick Cure by API. Went back a few hours later and all were at the top gasping for air. That made me realize that I had added the same meds to the tank a few weeks back when those fish started dying. I immediately changed 50% of the water to get the meds out, and as soon as the fresh water hit the surface they all were fine and acted like nothing had happened. I did lose 2 fish in the whole ordeal, but everybody else is swimming around just fine. Has anybody else had this experience with API meds? I have to say being that I have now lost a total of 8 fish in the past month from using this, I will never use this again. Water parameters were all in check. Ph 8.0, ammonia 0, nitrite 0, and nitrate about 5ppm. Temp is consistent at 78. Any ideas if it was the meds or what? I dosed exactly as the box said with one pack per 10 gallons.