Adult Daphnia are to big for most of my fish so one method is I use a turkey baster with two different size nets ,the first net catches the large ones the smaller ones go through ,and I've also bought some Monia which are much smaller perfect for Chilli's ,cpd and Medaka fry, my cultures crashed pretty quick when I was feeding them yeast , you need green water,you can buy a small bottle starter culture of ebay or make your own fill a bottle with water put a big pinch of that old flake food in and leave a desk lamp over it for 24 hours a day and it will turn green in around two weeks maybe sooner, stir it once a day for gas exchange