Frequently asked questions on aquarium pests

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Matt Clarke answers some of the most frequently asked questions on pests in the freshwater and marine aquarium.

There are loads of tiny transparent worms on the glass of my freshwater tank, especially near the water surface. What are they?

They're planarians - a type of turbellarian flatworm, found in tropical and coldwater aquaria. They generally live in the substrate, feeding on dead and decaying organic materials. Although they are not harmful to fish their presence in high numbers suggests that you are overfeeding and under-maintaining your tank.

Test the water and improve general maintenance by gravel cleaning more regularly and cutting down on the amount of food you offer.

I have some small anemone-like things stuck to some rocks in my coldwater set-up. They're 1-2cm long and semi-transparent, with long dangly tentacles. What are they, and can they harm my fish?

Your tank is infested with Hydra, a type of freshwater coelenterate related to marine anemones. They are usually found in ponds, but are sometimes introduced with plants or livefoods.

Hydra are predators and usually feed upon small zooplankton like Daphnia. Their bodies and tentacles are equipped with tiny harpoon-like stinging cells called nematocysts with which they spear passers-by. They rarely do large fishes any harm, but can sting and catch very small ones.

Hydra are sensitive to copper, so you can eradicate them with a copper-based disease medication. They tend to thrive in dirty tanks.

My freshwater tank is infested with snails which are damaging my plants. I've tried chemical treatments but they keep returning. What should I do?

Snail eggs are resistant to chemicals, making them hard to kill, so you'll need to treat several times in quick succession. Some fish, like Talking catfish, some Synodontis and most Botia species will eat snails, but only if there's no other food around. Even then, the pharyngeal dentition of most of these fish isn't powerful enough to crush big snails, or those which thick shells, like the Malayan burrowing snail.

You can buy a commercial snail trap, or you could make your own. Take two saucers of equal size and place one on the substrate about 30 minutes after you switch off the tank lights. Place a couple of catfish tablets on the plate and put the other saucer on the top. Use a small stone to prop up the top saucer so there's a gap about 5mm high. In the morning, before the lights come on, remove the stone, close the saucers together and lift them out - they should contain lots of snails. Keep doing this every night for a few weeks to keep the population lower.

I bought some live rock for my marine tank with what I thought were little anemones on. I've since discovered they're actually Aiptasia, and now I've got hundreds. Help!

Aiptasia are aggressive and will sting other invertebrates sitting nearby, which may result in tissue damage and can prevent those under attack from fully extending their polyps to feed normally. Once they start to reproduce they can be very difficult to eradicate.

Aiptasia can reproduce through pedal laceration, so if you try to cut, pull or scrape them off the rocks the fragments left behind can form a new anemone.

Squirting neat calcium hydroxide or boiling hot water into or onto them using a hypodermic needle can kill them, but unless you do this very regularly they will quickly recover.

Some reefkeepers introduce butterflies, like the Copperband, Chelmon rostratus, or Peppermint shrimps to their tanks to control Aiptasia. But there is a risk that they'll nibble the polyps of desirable inverts as well. The nudibranch Berghia verrucicornis is a coral-friendly natural predator, but sadly it's not one that's widely available in the aquarium trade.

My inverts are covered in flatworms. Are they harmful?

There are nearly 2000 species of flatworm but only a handful pose a threat to the health of invertebrates. Of the three main groups the majority are harmless scavengers that feed on detritus, decaying organic materials and the mucus of corals. Some can be harmful, but most outbreaks just look unsightly. Callionymids, like the Mandarins and Scooter blennies may eat them.

I have bristleworms in the substrate of my reef tank. Are they dangerous?

Dozens of species of bristleworm are found in marine aquaria. While they've long been regarded as a threat to corals and other inverts, it's quite likely many are actually pretty harmless and may be beneficial, especially in deep sand bed and plenum systems.

But if you inadvertently touch or pick up a bristleworm you'll soon know about it. They are covered in tiny, hair-like bristles called chetae, which stick in your skin like the spines of a cactus.

Unlike cacti spines, those of some bristleworms are hollow and filled with a venom, causing a mild skin irritation. Initially, the spines just feel a bit irritating, but if you are sensitive to the sting, it can soon turn into a dull, throbbing pain or a burning sensation. Pick the spines out with tweezers and soak your fingers in very hot water. Don't rub your eyes (or go to the toilet, gents) if you've just been stung...