I had a whole tank crash while using bio pellets. Tanks had been running for 5+ years, full of SPS and LPS. After I started the pelletsat half the recommended dosage and things seemed good. Then after my nitrates and phos started to come down I increased the amount of pellets to recommended dosage and thats when the STN began, after that RTN, finally everything went.
I feel in my tank there was a byproduct, or bacteria, that was released back into tank as the pellets were consumed. I say this because I couldn't get my Ph above 7.9, usually it ran at and now runs again at 8.3+, the water had a greenish tint no matter how much carbon i used or how aggressively I skimmed, corals got lighter in color and showed minimal polyp extension until eventually they expelled their zooxanthellae.
I woud like to experiment with pellets again to really figure out what happened but honestly I think carbon,GFO, refuge, and good old water changes are the way to go.
Liquid carbon dosing seems to be much more stable in my experience although you can have problems with that if you dose to much. I guess its like anything in this hobby, only bad things happen fast!