Daphnia should be stored in a cool place or refrigerated in the unopened bag they are sold in. It is advisable to use as soon as possible but daphnia should keep for up to a week if stored properly.
Feed daphnia only to freshwater fish. They can be added to the aquarium complete with the water they are supplied in or you can filter them through a net and add just the animals.
As a rough guide, a bag of daphnia contains enough animals for one feed for an aquarium larger than 24 inches – depending on the type and size of fish.