Daphnia (water fleas) are small freshwater cladoceran crustaceans up to about 3 mm (some to 4-6 mm). Moina is a smaller cladoceran (adults 0.7-1 mm). Protein is usually about 50% of dry weight; adult fat about 20-27%, juveniles 4-6%; value depends on what they were fed.
For: Small fish and fry. Adult Daphnia are too big for the tiniest fry; young Moina (under 400 µm) are small enough for many first-feeding fry.
Careful: Do not add adult Cyclops (cyclopoid copepods) to tanks of tiny fry: they prey on fish larvae. Copepods can also be intermediate hosts of fish parasites, and wild-collected cultures can bring in hydra and insect larvae that eat fry, so prefer clean cultured stock. Frozen daphnia leach nutrients quickly once thawed.
Examples you may see in shops: Live Daphnia, Moina (a cladoceran like Daphnia). Not endorsements.
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