Hmmmm, what are water parameters? Im thinking if that many fish died, ammonia/nitrate spikes...On top of anything that could of hitchhiked in with the feeders.
I'm willing to bet you caused a big ammonia spike when you added all the feeders into the tank. Also, you said your nitrates read zero, this, to me, sounds like you have no beneficial bacteria colonies and your fish are suffering from ammonia poisoning.