| From some quick research, the main use of a refugium for freshwater tanks, and I must admit, there is not a lot of info on this, is for growing plants in a tank to get the benifits of them sucking nutrients from the water, while still having the ability to keep fish that would otherwise not be compatible with planted tanks. I am not sure at all about if you can grow the daphnia in such a small tank (relatively speaking I guess) or even where to start looking to see how this is done. I would also think that the very small size of the daphnia would mean that they would be constantly sucked up by any pumps or filters you have on or in the tank and that may be a problem. If you could create a method of having two tanks, one display and one for growing daphnia, with the ability to turn on a pump whenever you wanted to move the daphnia into the main tank for feeding, then that might work.
Again, I haven't found much informtaion on this and so it is all pure speculation on my part with a sprinkle of logical resoning, which who knows how much logic I can really come up with anyways!?!?!?!? LOL | 
