Help High Nitrates

[danny zubot]

I have been having difficulty controlling my NO3 levels lately. They have been 20ppm for about a month, and now are about 40 or 50 ppm after 2 water changes to the tune of 25%. Gone through the usual steps, regular water changes, skimmer maintenance and reduced feeding. I try to stay away from adding things like Cycle and other quick fix products, I like to get to the root of the problem. I'm wondering about the crushed coral I use in my sump, could this be a cause? I have heard of bad batches of salt which would co-inside with the NO3 problems but I haven't tested yet.