Welcome to Fish Lore Amc...according to your aquarium information you are unaware of the
nitrogen cycle. If that is still the case then the link below will be a great help to you.
The Aquarium Nitrogen Cycle
Sorry to hear that you're having fish loss and difficulties. Are the rocks a new addition? Are they painted?
If you could tell us how many fish you have in your tank and what kind they are.
Sorry for all of the questions. The more information you can tell us the more we'll be able to make suggestions.
Best of luck and enjoy the site!
Ken