Thoughts on unnexplained (and very sad) die-off

[gregzz4]

Sorry to read about your losses

Sounds like a cascade effect
Low Sg starts killing inverts and other sensitive life, ammo levels start to rise, bacteria starts to die off faster, more ammo, fish who are OK fighting off whatever is present in good water start to succumb to things their lowered immune systems can no longer deal with
Add the Prazipro and possibly this was the final straw

Prazipro can raise pH. Ammonia from die-off combined with higher pH can be lethal at much lower levels than our target pH would normally allow us to get away with

Just my thoughts

As far as proceeding goes, have you been testing for a cycle ?