I've been neglecting my tank a bit, but I finally got around to doing a w/c after 2 or 3 weeks. W/c was 50%, added prime as usual....i haven't changed anything in my tank for months. And I've had the angelfish in there for nearly a year. Right after the w/c all my fish started swimming upside down and some laid on the substrate as if they were dead. I checked ammonia and it was 0, so I don't think there was ammonia building up from compact sand. ~30 mins later all the fish seem fine...but I really want to know what happened so it doesn't happen again. Was there something wrong with the tap....did I go too long inbetween water changes? Skipping 2-3 weeks for just one time doesn't seem lethal to fish.