Rather than a check valve that doesn't work properly, you could put in a vertical open pipe at some point in the horizontal pipe between the pump and the waterfall tank. The open end of this pipe should be above the surface of the waterfall tank.This allows the siphon effect you have now to be broken.

If you are using the waterfall tank as a filter this is important since gunk in that filter returning to the pond can contaminate it.

It just occurred to me that perhaps you plumbed it so the water enters the waterfall tank from the bottom. If that is the case, the above solution won't work. You would have to install a manual turnoff valve.

Thanks for responding,

Your first post really confused me. Thanks for clarifying. I have started to just shut the valve that I have in the pipe off when I turn off the pump to clean the skimmer. It's a pain but it works. I still would like to know of a good quality check valve.