Sucks Drew...sorry to hear. Crazy.
I would check electrical conductivity, maybe you have stray voltage in the water?
Or someone turns into a werewolf at night and beats the crap out of the others? Bristletooth to huge compared to others, he would be my guess if this is what is going on.
If you don't see any signs before, rapid breathing/scratching and no spots etc, seems you couldn't have an external parasite. So hypo wouldn't do anything? Maybe I'm wrong?
Dr Tim's is the bomb, I used it. It looks and smells like sewage so it's gotta be the real deal. Probably the most expensive poop in a bottle you can buy, I would use it sparingly since a little should go a long way (as long as you have media for the bacteria to colonize on).