A rock wall must be very well sealed to stop water seepage. ANY crack or crevice could leak. Most, what we call garden ponds, have a rubber or plastic liner put in to hold the water then the rocks are placed around the edges to look nicer.

If your rock is just stacked on a dirt bottom, the water will simply go under it and rise to the same level on both sides of the wall unless again, the rock is completely sealed and the bottom of the pond was compacted with clay AND the wall was compacted deeply and on both sides of the bottom.

Bullhead catfish will eat other fish. They will also eat their own babies. How big your filter will need to be depends on how many fish you have. More/bigger fish need more/bigger filtration.