In a fully cycled tank, Ammonia and Nitrite should both be at zero
So your tank does not seem to be completely cycled and that might be why a fish died
In a fully cycled tank you will have some Nitrate: but that is not a bad thing as it is not nearly as toxic to fish as Ammonia and Nitrites are..
Besides, plants need some Nitrate to grow
BTW over feeding can case Nitrates to spike as also can overstocking
So, water changes, after your tank is fully cycled, are mostly just to keep Nitrates down to an acceptable level.....for both fish and plants, so both can live in harmony in a tank...
Plants can also help in keeping Nitrates in check... particularly fast growing ones
Bottom line, an aquarium is a very small closed ecosystem and all things in it have to work in harmony with each other.... just like in nature.
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