There are two very different fish diseases that get called ‘black spot disease.’  One is caused by healing after injury (most commonly ammonia burn), and the other is caused by a parasite that infects the intestine of birds.  This second type of black spot disease is rare in fish tanks, but more common in ponds.

Parasite life cycle

The reasons behind the scarcity of black spot disease in aquariums can be found in the life cycle of the parasite that causes the disease.

The eggs fall into water in the droppings of birds that have eaten infected fish.  About a month later they hatch, and are called miricadia.  Think of them as little hairy egg-shapes that move the short hairs in unison to swim.

The miricadia swim until they encounter a snail, into which they burrow and develop.  About 6 weeks later, cercariae emerge.  These look like little tadpoles with forked tails, and their aim is to find a fish.

When they find a fish they burrow under the skin before losing their tails.  Now they are called metacercariae, and they protect themselves in the fish with a hard cyst that contains dark pigment.  These cysts appear as small black spots and give their name to the disease.

The next stage is for a bird to eat an infected fish, where it develops into a flatworm with a shape a bit like spoon with a broad handle.  Although the parasite feeds off snails and fish, it is on the bird that it places the greatest burden.

It is in the bird that they consume the resources necessary to reproduce and lay countless eggs which then fall into water in the bird’s faeces.

Barring highly exceptional and unlikely circumstances, the only way for parasitic black spot disease to get into an aquarium is when infected snails are introduced.

When it comes to ponds, however, the chances of infection are much higher.  Any visiting heron or kingfisher can cause and outbreak.

Effects on fish

Fish infected with parasitic black spot disease will usually display signs of iritation such as rubbing and flicking of the body.

The black spots themselves appear as small, dark, well defined, raised areas of skin about the size of a pin head.

Lasting damage is rare, although blindness can occur if the carcariae end up in the fish’s eye in large numbers.

Other than that, there is no cause for alarm when healthy adult fish are infected.  Young fish can be killed by the disproportionately high requirements of the parasite on their bodies, but the parasite will not complete a life cycle if it kills the fish.

It is worth noting at this point that on the rare occasion of aquarium fish becoming infected, the number of black spots on each fish will likely be huge.

This is because there is a high number of fish that are not eaten by birds.  So the flatworm needs to produce as many eggs as possible, in order to infect as many snails as possible, in order to get lots of fish, thus increasing the chance of an infected fish being eaten by a bird.

Miricadia on a snail that enters an aquarium will emerge to find they have no trouble finding a fish.  Not only that, they will all attack the same few fish.

Treatment

Other than removing snails, there is no other treatment necessary in an aquarium.  Providing the fish in the aquarium do not get eaten by a bird, and the same bird does not return to the aqarium a month later to deposit faeces in it, the parasite will die off.

In ponds on the other hand, it will probably be necessary to purchase treatment for the disease.  Copper sulphate can be used (only under licence in some parts of the world) to kill snails, and many commercial treatements are also available that target the parasite.

Although impossible to prevent, it is also wise to discourage herons and other fish-eating birds from visiting ponds.

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