It's happening again! One week after I lost two clown loaches immediately after a water change I'm losing three more. They're still alive, lying on their sides, breathing normally. One is making periodic dashes to the surface, but the other two aren't moving.

I am 100% sure I remembered prime this time.
I am 100% sure I matched the temperature well this time.
No signs of distress at all from the danios or the pleco or, for that matter, the other three loaches.
I have changed 4 other tanks this week without incident so there is nothing wrong with my source water.

I don't understand it! My poor fish...