The way I did my Daphnia was to 3/4 fill a bathtub with water, I used tap water and waited a week and put in the daphnia culther and chucked in a banana skin and left them to it, from time to tim, like a week I would chuck in a pinch of grass, lawn clippings will do. when I couldn't see the banana skin, I would chuck another one in.
I have chucked in a whole rotten banana, and found it infested with red wormy things after a week, used those to feed the fish too.
Oh, if you can keep the bathtub partly shaded during summer would help too, otherwise it gets too hot and the daphnia will cook