You don't really have to do anything special, especially considering daphnia aren't really anything exotic (you can find them in many ponds etc.) depending on where you live.
A very minimalistic setup is very easy to achieve – I actually did it for a simple test, because I was also very interested in the whole concept of having a somewhat stable water tank. You can also use this if you have plants you can't water using tap water, etc. It's also an interesting experiment for kids rather than using some kind of shrimps.
Anyway, the initial setup is pretty straightforward:
- Take a small tank or plastic box that may take 2-10 l (size is really up to you; my experiment here was a 5 l box meant for food storage). You'll want some way to close the top so nothing escapes. :)
- Fill it with aquarium water.
- Put it on a windowsill or another sunny place.
- I added one or two tiny branches of Chickweed for "asthetics", but that's really optional.
- If you don't have plant bits from your aquarium, let it stay a few days in the sun. This gives algae some time to grow.
- If you have beach almond leaves, add a tiny bit to avoid fungi and bacteria growth.
- Add a few bladder snails (2-3 per liter are probably enough), ramshorn snails should work as well. The population will balance itself.
- Add the daphnia.
- Just feed the snails once every or every second day with regular fish food or tiny amounts of spirulina. If you use spirulina, make sure to not overfeed; ramshorn snails won't like the water dirty.
- From time to time add some fresh water and potentially some chalk (e.g. tabs sold for shrimp aquariums).
The concept of the tank is pretty simple:
- Sunlight causes algae to grow.
- Snails eat algae and other food you provide.
- Daphnia feed off "dirt" produced by the snails.
This way you won't have to directly feed the daphnia, but you can create a stable and "flourishing" tank. If you prefer feeding the daphnia directly, basically do the same, but skip the snails and feed them drops of condensed milk.
Basic rule of thumb: While the water isn't clear, you don't have to feed.
Her's a quick shot of my tank in early spring – just after feeding the snails). Most daphnia were still inactive at the time, hiding between the algae on the ground. I thought they didn't survive the winter on the windowsill, but all emerged a few weeks later (unfortunately don't have any pics at hand).
The tank will sooner or later become at least a bit green in some way. That's completely normal. You can remove excessive algae, but I'd always leave at least some for the snails.