I was unfair to tilapia. It was likely merely coincidental that after the addition of the tilapia, the damage started to appear on the snail bullhead. That was actually coming from a yellow bullhead, who used to be beta and the snail alpha but it was changing apparently (in retrospect) right when tilapia was introduced.
One's a fluke, 2 a pattern, 7 a law. We lost 7 bullheads over the last 5 years, from 3yo to 5yo to seemingly the same bug, as can be judged by the same symptom of great swelling and overall length and progress of their last illness, supposedly a bacterial pathogen, maybe columnaris that infected our systems and fish in 2018. The final victim is the 4yo 15in/37cm yellow bullhead.