Ive had a quick skim read through your historic posts to see where you think your tank might not be working well. Nothing particularly stands out as a problem. What issues are you having?
The gravel looks a little course. It will trap detritus, and potentially give you water quality issues. The water test results you provided in January showed no problems water quality wise though. A little ammonia, but that could easily be down to inaccurate testing. Some gravels might be calcium carbonate based and increase the pH, general hardness(GH) and carbonate hardness (KH). That water test doesnt show high pH or KH, although your GH is high. I dont think the high GH is down to your gravel though, or the pH and KH would also be higher.
I dont see a need to change the substrate unless you can tell us something you havent previously. If you want to for appearances sake, then change a little at a time. Do 25% a week, over a month. Changing substrate will cause you to lose some cycle, so do a little at a time to minimise this effect.
