If draining is not something you want to do, I believe you can install a siphon drain system instead to avoid using the spillway. While my puddle exclusively uses a spillway, there is little chance of it washing out due to heavy vegetation and a slow slope. If there was more risk, a siphon system would be added, with the pull from the bottom.
I have seen two ponds that had a concrete spillway where water worked underneath it and washed it out. This is probably more common in cold climates where frost will heave most anything, leaving an easy gap for water flow. Spillways are risky.
But I would agree with others here and learn as much as you can about the pond before making any more moves. With aquariums, small incremental changes are preferable. Large changes like adding too many fish or cleaning too much at once can destabilize the tank and lead to disaster.
