I "pond dip" to feed my hatchlings - I have a 1ft plastic tank; I dip it into a corner of the pond and pour back in through a fine net. What's left in the net I put in with the hatchlings.
If you don't have a fine next - you can pour the water slowly out of the tank, the daphnia seem to swim against the flow; do this until there is bit of water left and it'll be packed with water fleas.
Cooler the weather, lesser the water fleas.
Down side to this; you end up putting a little dirt in - and I suppose there is a risk of introducing some potentially harmful water monsters in with your hatchlings.
But I raised my last batch this way and didn't have any problems.