Hydra in an Aquarium: Harmless to Fish, Deadly to Fry

Hydra in an aquarium only matter if you have babies in the tank. They are tiny relatives of jellyfish that sting and eat whatever drifts into their tentacles. Adult fish and adult shrimp are too big to be prey. Fry and baby shrimp are not.

In a community tank, cut the feeding and they starve out. In a shrimp or fry tank, cut the feeding and treat if they do not go.

What hydra look like

A hydra is a thin stalk, a few millimetres long, stuck by its base to the glass, a plant or a rock. At the free end is a ring of four to six thin tentacles that sway in the current. They are usually white, pale brown or bright green.

They do not crawl like a worm. They stay put, and contract into a small blob when touched, then slowly stretch out again. Over time they can move by somersaulting end over end, but you will rarely see it.

If what you have is moving, it is not hydra. The worms in a fish tank guide covers the things that do move.

Why hydra appear

Hydra need a lot of very small food. That is why they turn up in two kinds of tank above all others: fry tanks, where the keeper feeds baby brine shrimp or powdered food several times a day, and shrimp tanks, where fine food settles on every surface.

They were almost certainly in the tank already, in numbers too small to see, having come in on a plant or in shop water. When feeding goes up, they can cover the glass within a week or two.

Hydra in a community tank

Feed less and let the fish help. Feed what the fish finish in two minutes, vacuum the substrate at the next water change, and within two or three weeks most of the hydra shrink and disappear. Many fish eat them, including gouramis, guppies and mollies. There is no need to medicate.

Hydra in a shrimp or fry tank

No Planaria, a betel nut extract, kills hydra as well as planaria and is the usual choice in shrimp tanks. Shrimp tolerate it at the label dose. It is sold as safe for snails, but keepers regularly report snail losses, so move any snails you care about first.

Fenbendazole is even more effective. Hydra are more sensitive to it than planaria are, so it often works at the low end of the dosing range. It kills snails, though, so use it only in a tank without them.

With either treatment, follow the product's dosing exactly, add an air stone for the duration, and do a large water change afterwards. Many keepers repeat the course a week later to catch any hydra that survived as buds.

Keeping them out

Hydra arrive with live plants, shop water and live food cultures. Tissue-culture plants are grown sterile, and pouring fish into a net rather than tipping the bag water into the tank keeps the shop's hydra in the shop. Rinse baby brine shrimp in fresh water before feeding them.

The one thing worth taking from this

Hydra are a measurement of how much small food is in the water. Bring that down and most of them go. Treat only when there is something small enough in the tank for them to eat.