Diagnose slow decline

[daniella3d]

Just my guess but lack of nutrient seem like your problem.

I would be worry about that thick sand as it may become anaerobic eventually and release some gaz in your tank.

There is not enough liverock to support good planktonic life. Try to feed zeovit amino acid and coral vite every other day. Pods eat phytoplankton so feeding the tank with live phyto will quickly bring bac the pods if any are still present. Of course stopping the skimmer for an hour while feeding phyto is required.

Your tank would be prettier with a bit more live rock. Now it looks empty and as if it's missing something, some substance. Try to find some nice shaped liverock (fully cured of course), or some nicely shaped dry base rock.

Also, I would never use carbon full time as you are removing some trace element from the water. There is no need for full time carbon treating. You would be best to use a skimmer only, no carbon.