The wire mesh idea is one of the best suggestions. My father and I had twelve Koi goldfish in our Japanese garden pond. The pond had a large cascading waterfall. The suction of water required to fill the waterfall was quite strong.
To keep the Koi goldfish from being sucked in by the current generated by the pump we covered the intake of the pump with a wire mesh. That did the trick.
The wire mesh filter must be cleaned on a regular basis or the pump will burn out if it does not have an adequate volumn of water coming into the intake.
I have written quite extensively on Japanese gardens on my Hub.