another RO/DI question

[Lampshade]

Just water polishing. For drinking water 0 TDS makes water taste like air. Everyhing else should be taken out by the time it gets to the DI canisters, all that should be left is minerals. In drinking water haveing a small amount of minerals is not the end of the world, or unsafe. For a fish tank a small amount of iron or copper can be very bad.