I did this on a small scale for a couple of years and this is the first time I tried it with a decent sized pond. It's been working well so far. The waterfleas process the pondwater that comes into the tank and they produce hordes of babies (too small for the fish to eat) that get washed into the pond and these babies also feed on algae until they become big enough for the fish to consume. Also some adult waterfleas (Daphnia which used to be sold as fish food) get washed in to the pond too, also some shrimp end up as fish food. It has been a good system so far.