I have a pond that consist of three basins. Water is pumped to the top
pool and then comes down to the lower pools through sluices. I
finally got the pond operating and the leaks minimized, when my wife
decided we needed fish. We placed the fish in the middle pool, when the
pond is operation the fish get caught in the current and carried to the
lower pools.

The sluices are about 12" long, the width of a 2X8. The water is between
1" and 2" deep depending whether it is from the upper pool or the middle
pool.

How can I contain the fish in the middle pool?

Every thing I have thought of either does not work or creates a dam,
upsetting the dynamics of the pool.