Wow! I think your nitrates could definitely be your problem. Things to do for nitrates that work: a large ball (I would guess at least the size of a basketball) of chaeto with bright PC lighting about it so it grows fast, efficient protein skimming, a remote deep sand bed in the sump, and of course regular water changes. Until you get your nitrates way down, I would do at least 20% water change once, maybe twice per week.
With well established sea anemones, nitrates don't necessarily spell disaster. I once had nitrates over 60 with two H. magnificas--and I freaked when I realized how high it was. Frankly, it was always low when I tested, so I stopped testing--which was a mistake I suppose, but I still don't test often. Anyway, my mags didn't seem to be as happy as usual, not expanding as large, but they looked okay. Once I discovered how high my nitrates were, I began doing the water changes. Bought some chaeto. And tuned up the skimmer to skim faster with a lighter color skimmate. It took a few weeks to get them back to undetectable.
Out of curiosity, what salt do you use? Also, what is your specific gravity and temp?