The daphnia are a very nice early food for young fish. I raised my own for a while but eventually had a problem maintaining them because the new water always killed the colony off. Daphnia are very sensitive to copper compared to fish and can be killed quickly by any minor trace, even the small trace that comes from water passing through copper pipes in your house. It will not take long for the daphnia to die of starvation if you don't feed them. If you can find a source of green water algae, you could just arbitrarily put some into your water as an almost ideal food for the daphnia that will not harm the fry. Another daphnia food that will work but you need to be careful feeding it when you have fish is brewer's yeast. Unfortunately the brewer's yeast can pollute the tank if you are not careful with the feeding amount. One of the benefits of the daphnia living in the tank is that they will reproduce rather quickly and newly hatched daphnia are a great size for newborn fry to eat. As the fry grow larger, they will eventually get big enough to eat adult daphnia. Daphnia do not usually use sexual reproduction when they are well fed. Instead they more or less clone themselves. This means that they can reproduce fairly quickly with an adult, 5 or 6 days old, producing lots of babies every few days.
