Noto;3474161; said:I meant metals from fish food, not from water. Looking at the back of my TetraMin can, it lists cyanocobalamin (copper-containing), manganese sulfate monohydrate, zinc sulfate monohydrate, ferrous sulfate monohydrate (iron-containing), and cobalt nitrate hexahydrate, as well as the fish-toxic preservative ethoxyquin. Many of these are toxic at modest concentrations and should be removed via occasional water changes.
You are right though that topping off but not changing the water would allow minerals present in the tap water to reach higher concentrations (because the minerals do not evaporate). Just another reason not to give up on water changes altogether.
As if we needed another reason not to use TetraMin...