Matt,
I have had my sand bed cause problems twice. Basically, here is what happened. After I have fed fish very little for a while and did large water changes, each time, phosphates came back in about 2 days. Did use a GFO reactor.
After trying everything, I removed 1 inch of sand, from all parts of tank. I first vacuumed it, and later just used a strainer.
I noticed huge drop in phosphates the first time around, and the 2nd time around, 6 months later, nitrates and phosphates went back to 0.
After contacting BRS tech support, we chatted about my generalization, that people with very deep sand bed did fine. People with 1 inch of sand same. It's the reefs with 3-4 inches ( meaning not very deep ) , where you are kind of in the middle is a problem.
This is just my opinion.
However, for me, my parms, went quickly to 0, each time I removed a layer of sand... That is not in dispute ( for my tank... )