I'd also be concerned about the feeding habits. Once every other day isn't going to provide the proper nutrition all those fish need. I wouldn't be suprised your fish are being murdered; they're all starving so they could've resorted to canabalism. Try feeding at least 2 times a day, if you're not feeding more often because you don't like performing water changes, hey, it comes with the hobby!
This is highly unlikely. Fish have slow metabolisms, once every other day should be fine. If anything, I would boost the ammount of time it takes them to consume the food to two minutes, but again, fish don't need to be feed 2 times a day or even once a day. Goldfish, for instance, need to be fasted(sp) for a few days.
LFS's sometimes are really laxed about cleaning out their test tubes which throws the reading way off. For example, if I don't completely and thoroughly clean out a test tube that I have used to test ammonia, then my pH is read completely off, (like 8.6 compared to the 7.4 it really is).