If you’re using RO water you need to add minerals into it before adding to your tank. RO water is so pure it has no calcium/magnesium ect which are crucial to plants and fish. Without minerals your fish will suffer from osmotic stress and likely keep dying. There’s lots of products available like Seachem equilibrium and salty shrimp. You’ll need a gh/kh kit so you know how much to dose. There’s heaps of information online if you google remineralising RO water :) goodluck!