My previous tank had high nitrates too and I had a sand bed then. My current tank is BB and I was getting NO3 up to 50! Started dosing NOPOX and got it down below 10. That being said, I don't think sand is 100% cause to high nitrates. I am using TBS live rock, and have a high rated skimmer pulling tons of gunk. Still amazes me that my nitrates got that high, I even had my LFS check my water too. I have 10 fish in a 100 gal.O started this tank with 50/50 live and dry rock and the sand was "live" and I rinsed the sand. It's 6 months old. the cyano wasn't the reason I removed the sand. The reason was my nitrates were at 40 and I have a super over-sized skimmer and siphon the bed every water change. My nitrates dropped after I removed the sand and I also noticed the cyano did not return. Oh my gosh the sand was so nasty that I took out even w/ regular maintenance.