[Myka]

How old is this tank? If it's less than a year old, what type of rock did you use to start it?

These guys are right about the phosphate. The key element they haven't mentioned is that if the PO4 is "zero", then the NOPOX will not work (nor any other type of carbon dosing). The bacteria that feed off the NOPOX use organic carbon (the NOPOX), nitrate, and phosphate. If any of these nutrients are missing then the bacteria will not be able to do their job. If you've stripped the PO4 out, then the nitrate will continue to rise.

The rock, sand (for those who have it), and the water always want to be at equilibrium - so when you do a big water change, the water will be cleaner than the rock and sand. The rock and sand will then leach both nitrate (and phosphate if available) until they are back at equilibrium. That's why you're seeing no reduction in nitrate the next day. For this reason, using water changes to reduce nitrate is not a good long-term solution. The NOPOX or biopellets or something is a good option, just stick to 10-15% water changes, and you may need to ADD phosphate to your system in order for the NOPOX to be able to reduce nitrate.

So first order of business, get a phosphate reading. I also agree the phosphare checkers are a good option. I don't think you need the ULR (ultra low range) one since you want phosphate to be high end to get a reading on the ULR and the regular, so it really doesn't matter which one you buy.