What is the best way to treat high levels of nitrate and nitrite? I haven't had this problem YET but I want to avoid this problem all together. I have been doing regular water changes and avoiding over feeding them.
A fully cycled tank shouldn't see nitrite at all... Nitrate on the other hand can be kept under control through water changes. That's really the main way non chemically to adjust it, keeping live plants will also lower your nitrate number as the plants use some of that nitrate as fertilizer.
If you are cycling with fish then every other day water changes are the best bet for keeping the nitrite under control