Did a water change on my clown loach tank just now. Came back 45 minutes later to feed them and 2 of my 8 loaches are stone dead. No signs of distress whatsoever from the survivors, nor from the dozens of danios and the pleco in that tank. Water parameters check out just fine, 0/0/15.

I'm pretty crushed. What did I do? Did I forget to add prime (I have since added a double dose just in case)? I can't say with complete confidence that I remembered. You would think other fish would be showing signs of distress... or that there would be an ammonia reading. Temperature variance? I was keeping an eye on that as I refilled the tank... and if there was one, the heaters got it back up within that 45 minute window. What else could I have done?

Really upset. These fish grow so slowly, I've had them for a year and I was just admiring the size they put on... and now I just killed two of them somehow.