I have gravel and large rocks on the bottom of the tank, and I look at it everyday. If I fish died, there's no way it was dead more than a day without me seeing it. Plus, if that's it, what is killing them initially?! I mean we're talking like 2 inchx3/4 inch fish here roughly...

I did just buy a video camera...maybe I should set it up, that seems like the only way I'm going to know for sure what is going on. LoL