I am sorry to hear about your losses.
I remember reading about different types of rock in my "Conscientious Marine Aquarist" book that some rocks like lava rock can contain heavy metals in it. They may not show up right away, but sooner or later it will rear its ugly head when you least suspect it. The best way to fix your pH problem is to use live rock. Tangs love to graze on them all day. In addition, Tangs are very suceptible to low levels of pH, especially anything below 8.0. Also, if you are using glass tops I would leave the flaps open. This was a real problem for me, so I got rid of the glass tops all together and went with eggcrate.
You can usually lower nitrates by doing water changes, and cutting back on feeding.
I have never had luck keeping an anemone in a tank less than 6 months old. What type of lighting did you get for your system?
I would slowly work on getting your SG
raised to 1.023 as well.
I have seen tests out there for iron. Not a bad idea.
Damsels can survive just about any condition of saltwater. Very hardy fish.