Depending on where you live and how cold it gets it's a minimum of 3 feet to winter over fish. At least that is what I read when I dug my pond. I know people keep goldfish in horse tanks that live year round. They do have heaters in them during the winter though. Really it's just giving them somewhere to hide from the ducks that will save them imo. Put a couple cinder blocks or bricks in the tank and they should be able to hide in the holes to avoid being eaten.How deep is Deep enough?
I think Mine would be too shallow