it happens.. i did a WC on my gf nice planted community tank a while ago... even added prime... super funky "no one knows why" tapwater managed to kill off 3 congo tetras, 1 oto, and put one heck of a hurting on my L239 (fancy pleco)... i spent like 4 hours holding my pleco infront of a powerhead in a different tank trying to keep him breathing... he made it but he ended up with a MASSIVE case of ich (my first ich outbreak EVER)...