Same as Gill 10L is fine. This is the method I use for Daphnia culture:Is some 10L bucket fine or something bigger
I use ordinary 99p Builder's buckets, with a handful of limestone gravel, filled with rain-water, and with a good handful of grass clippings. I leave this a couple of weeks and then inoculate it with some Daphnia and a couple of pond snails. These are reasonably good and also produce Blood-worms and Mosquito larvae. As the productivity falls off you can just add some more grass clippings. If you use sheep or rabbit manure rather than grass they are more productive, but may crash. The same with positioning in the spring they can have full sun, but as the days warm up they need to go to somewhere with more shade (especially around mid day). If you want Mosquito larvae you need to float a cork in the bucket to provide a landing platform for the mosquitoes. If you can stop these freezing solid, they will produce Blood-worms and Daphnia even in the winter.
You can feed the cultures with a gram flour and paprika suspension, your also meant to be able to use bakers yeast, but when I tried this it all went horribly wrong (too much yeast?). I've cultured Daphnia inside using 1 litre ice cream cartons with very gentle aeration (you need big bubbles rather than the small ones from an air stone), a snail and the gram flour feed mix. They are quite high maintenance to keep going productively as they are very prone to "crashing", so you really need to keep multiple cultures, seeding new ones all the time. You can harvest every day from them when they are going well.
If you don't mind them not being very productive you can use an ordinary rain-water butt and just inoculate and then ignore them. The advantage of this is that you trickle feed them with water changes and the numbers never build up hugely into the "boom and bust" cycle. It should also give you a reservoir of Daphnia for seeding your buckets. Rain-water may not work for everybody, but It is all limestone around here and even my rainwater is above pH7.