I just don't see your sand being the issue. The BB that consumes the ammonia in the tank yields the nitrates, so if anything, it seems like a healthy tank, and the water changes keep the nitrates low. I would bring the sample in and pick their brain. My LFS
said that it's hard keeping the nitrates at "0", which is why you would do water changes. And he wasn't even too concerned when I was at 10, but he was surprised that it didn't take long to get to 10 in my tank. Point being, is that it may just be how everything goes in your tank, and maybe 5 isn't as bad as you think??