I've never used cycle supplements, most need to be refridgerated to be kept a live. There is one that has spores in it and doesn't need to be refridgerated and is suppose to be pretty good.
Ammonia and nitrite is strees full to fish so some people use cheap fish to do this that they don't care if they die or shorten the life span.
If your tank has been running for 2 months with fish it could be cycled, you should get test kits to test your water. You can also take some water to the fish store and get it tested. I only have a nitrate test kit cause once your tank is cycled this is the only thing that builds up.
I brought this up because it may have been what killed your fish early on.
Reading a sticky or two about filtration and the nitrogen cycle should answer all your questions