I feed my successful daphnia culture every 3 days or so on nothing but a cultured yeast mix. To do this I add 1 tsp of dried active yeast to about 30mm of water in a tall glass. Mix this with a fork until all yeast is disolved, and fork it vigourously to aerate the water. Continue doing this every 1/2 hr or so for 2 or 3 hours, then feed to the daphnia. For small ponds (eg old washing machine bowl) use 1/2 tsp yeast to start with.