Hey Jason,
Sorry to hear of the trouble. That a lot of trouble with that volume to deal with!
I've found the AIO biopellets remove too much phosphate. In order for biopellets to remove nitrate there has to be phosphate available. In order to remove phosphate there has to be nitrate available. The plastic in the biopellets is the carbon source, which also has to be available. I'm thinking that what may have happened is the AIO biopellets have stripped all the phosphate out of the tank which will allow the nitrate to climb and climb. I think most people are having better success using either standard biopellets with some GFO in a bag or reactor, or using 1/2 AIO and half standard.
Have you tested nitrate and phosphate? Of course if you have a bunch of algae growing you will get skewed numbers, but I'm curious if phosphate is indeed undetectable? This would explain the Chaeto die-off as well. I don't think year old T5 bulbs would be the tipping point - there has to be nutrients for algae to grow in the first place. The corals looking pale and nutrient starved could be true, hair algae is like Chaeto, it's very good at reducing nutrients.
Also, what do you have in the tank for clean up crew, and in what numbers?