Howdy,
I have successfully kept daphnia colonies for years - in large pickel jars or 2 gal tanks. I cannot confirm what VOYTEK said, they lived in old water and did just fine. Their bowls were located on a window sill and overgrown with algae. However, daphnia do not graze but rely on suspended food. Thus, you need to feed them dried yeast. Just replace evaporated water or perform partial water changes when you siphon some of them out for feeding your fish. Daphnia need medium-hard water to thrive.
Best of luck.
HarleyK