Just my .02, I think your mechanical filtration needs a boost. If in fact it is the sand or if the current filtration is unable to keep ahead of the bioload or is for some reason running innefficiently, this is what immediately springs to mind looking at this tank. If this were my tank, I would immediately add additional filtration. I really don't think a tank can be "overfiltered". Given the expense that you have put into this tank, this would be worth it, if even for the short term until your biological filtration matures.
I had a similar situation recently when I moved. It was amazing, within two days the tank was nearly pristine. I gave the biological filtration some time to recover, and was able to remove the supplemental filtration.
It is a beautiful tank and I would love to see it at 100 percent!