Hey plecosword, what's that growing on the back wall in your first picture? Do or did you have a cyanobacteria outbreak underway? Makes me wonder if you might really have a nitrate problem despite your test reading. Do you have any way of getting a second independent measurement on nitrates and phosphates?
I'd also bring your salinity up over a week or so. I run mine at 1.026 which is better for the inverts.
Any possibility of a heavy metal, such as copper, being present in the water? If there are contaminants in the water, a poly filter might help in this case.