Seachem De-nitrate. One batch lasts forever (all it is is extremely porous pebbles that provide the proper habitat for anerobic bacteria - e.g. the bacteria that consume nitrate versus aerobic bacteria that consume nitrite - to live). No need to ever replace it, just rinse once in a while if it gets covered with detritus. And no way to 'over do it'.
Works like a charm. The key is to ensure the flow through the reactor is between 25-50 gph (as per the instructions).
Add a little mesh pouch of something like Phosguard in the reactor with it and phosphates will managed as well.