Live blackworms

Lumbriculus variegatus, a small (4-6 cm) aquatic oligochaete worm related to earthworms. Not the same as tubifex (Tubificidae), but worms sold as 'blackworms' or 'tubifex' can be mixtures of species.

For: Most fish that can swallow them, especially bottom feeders (catfish, loaches); also African dwarf frogs.

How to feed: Keep in shallow dechlorinated water in the fridge, changing the water after 24 hours and then weekly. Feed with a cone worm feeder (or similar) so they do not bury in the gravel; buried worms can survive and feed bottom dwellers later.

Careful: Tubifex-type worms (sludge worms), sometimes mixed in with or sold as blackworms, can carry parasites (cestodes, flagellates, microsporeans, myxozoans) and build up toxins from polluted mud; Tubifex tubifex is the host of whirling-disease myxozoan Myxobolus cerebralis. Buy cultured worms, rinse and keep in clean dechlorinated water.

Examples you may see in shops: Live blackworms (Lumbriculus variegatus). Not endorsements.