I was at work today talking about this and one of my buddies told me he witnessed his red tail shark swim right up the output stream of the HOB and into the reservoir area. That finally explained the disappearing fish for him. I laughed out loud when he said it, but it make a lot of sense. If you keep the water level in your tank up where it should be, there's really not that much of a lip that the water is flowing over into your tank. I've noticed a lot of fish like to play in the water flow and I can completely envision a fish swimming up the return into the HOB reservoir. Fish naturally traverse lots more challenging things than that in nature. I haven't used a HOB in a long time, so that possibility didn't occur to me. I'm glad he relived that one for me.