Helo Wonderfulge an Welcome!
What are your exact readings?
We often go to a
LFS and get our water tested and we get the usual " fine" LOL
Also, you mentioned using stress zyme, as shawnie said it contains the wrong kind of bacteria, making it seem ike your tank is cycled, but it is actually starving the good bacteria which is generating. You see, the bacteria needs the ammonia ( its kinda like its food

) so if something else is using up the ammonia ( Stress zyme) it wil kill it.
If you woud rather use a product that will speed up your cycle, then most of us reccomend TETRA SAFESTART it is the only bacteria in a bottle that will ACTUALLY cycle the tank.
Also, if your fish died of ammonia poisoning, then i would suggest you do 40-50% water changes daily to keep keep the leves down, as wel as adding Prime. Prime is a detoxifying product that will detox the ammonia and nitrites for the first 24hrs, so the enxt day when you do a WC you dose again, and so on.
Hope i helped
