Parameters seem alright to me although I wouldn't let nitrate get above 20 nor phosphate any higher. Alk couldn't hurt to boost a bit to 8ish but like anything everyone keeps slightly different with your levels being about min. I wonder if dead spots or lack of flow is a factor? Do you have wave makers? Your rock may be getting old(ish) and this is of my own opinion here and I don't mean trash it by any means. My tank has been running for 6 years and I've been having to do more water changes lately to keep nitrates at 11-13 ppm and phosphate .8-.10 and using phosguard with more sand/substrate agitation to stir up detritous and blowing rocks off some lately myself. Also, I've changed direction and pumped up flow on wavemakers a bit to prevent dead spots/settling of food/poop etc. I almost feel like my tank is going through a "turnover" type of phase as sometimes described in lakes so it's been a lot of work lately all of the sudden.