One rule applies, to calm the nerves of the hobbyist; most worms seen outside the fish are usually not internal parasites and not considered as harmful to the fish. Parasitic or harmful worms will not be visible on the gravel or the glass of the aquarium.
Noticing small white worms the size of a few millimeters crawling along the glass are either flat or roundworms. Flat appearances are planaria (flatworms) and the more thin and wiggly ones are nematodes (roundworms). Both of them do not exceed 4-5 millimeters. Both types are harmless to fish and thrive on excessive waste in the water column.
To free the tank of these worms simply do not feed your fish for 3-4 days as they will often feed on them.
The fluke is a parasite worm that is visible on the skin or gills of the fish. Measuring only 0.3 - 0.5 millimeters in length, this oval shaped worm can spread and often result in fish death.
Internal parasitic worms can not easily be diagnosed. Advanced stages can be visible by ulcers on the skin. On the other hand, not all internal worms do pose a threat to the fish.
First signs of internal worms are an increased appetite without weight gain or even weight loss while consuming increased amounts of food.
The tapeworm is the best known though not very common.
Worms of Concern
Anchor Worm
Young anchor worms are free swimming crustaceans that bury themselves into the fish’s skin. It takes several months before the worm becomes visible in form of holes or ulcers on the fish's body. After laying eggs, the worm dies off.
Since the worm can not be removed by hand, a potassium permanganate bath for about 20 minutes should cure it (dosage 10ml/l).
Thorny Headed Worm
Visible symptoms are white or green threads on the gills. The fish often scratches on objects in the aquarium.
The thorny headed worm is similar to the anchor worm, only smaller in size. It attaches itself to the gills. The cure is also a potassium permanganate bath for 20 minutes (dosage 10ml/l).
Flukes
The symptoms are mucus covered gills and/or body, red spots on the skin, fins appear eaten away, as well as rapid breathing.
Flukes are flatworms and are similar in appearance as Ick and can be better viewed with detail through a
magnifying glass. Flukes will destroy the gills and kill the fish if left untreated.
The best cure is a potassium permanganate bath for 20 minutes (dosage 10ml/l).
Threadworms
Threadworms are internal fish that sometimes emerge from the fish's anus. This parasitic infestation can be fatal if not treated in time. Preferred treatment is parachlorometaxylenol soaked fish food and a bath in the same for several days (dosage 10ml/ liter).
Leeches
These external parasites are visible on the skin, gills and fins of the fish and are similar in appearance to Ick.
Since they attach themselves to the fish, the best method of removal is a bath in a salt solution for 20 minutes (dosage 2.5 % salt to water). During the bath, most of the leeches will simply fall off; the ones remaining can be removed with a pair of tweezers.
Copper sulfate has been used to successfully remove and control less harmful worms. Over time, copper has been found to do more damage than good concerning the overall balance of the aquarium. The side effects of copper are rarely in relation with the possible benefit
