So I want to make a contained pond in my backyard, it will be 6long x 6wide x 2deep..... And my goal is to put the wild oscar I've seen in my neighborhood pond, my oscar from my tank, my red devil, and a bunch of convict fry..... and some babe bluegill for oscar food. I plan to filter it with fountainheads..... How can I keep predators from dipping into my fish stock?