Poor HD.
fishfanatic, I'm wondering a few things. For instance, is that skimmer properly sized? Have you tried letting the diatom bloom "crash" itself (use up available nutrients, much like lemmings, til it starves itself out)? I am sorry, but I know that you have given many rundowns before, your w/c regimen, parameters, full filtration, source water info, etc., etc., but I'm drawing a blank.
Have you got a refugium set up? If not, maybe it's time to consider it? Do you mess with the system quite a bit? I get the feeling that you're like I was with my first tanks, could NOT keep my hands out.. til I got yelled at by an old-timer. I can't post what he said, but it was essentially to "Keep your hands OUT of the tank!" (Many expletives deleted, though in truth it was most entertaining.)
I'm glad you're not giving up, but let's take things very slowly right now, time may be all this needs.