A Closer Look at Black Spot Disease – Part 2 (Parasites)

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.


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.

20 Responses to “A Closer Look at Black Spot Disease – Part 2 (Parasites)”

  1. sophie diaper Says:

    hey.. i have got a goldfish and its started to get sort of black spots on it but the spots are very small and alltogeher on one side of the fish. this white stuff also started coming out of its mouth too. can you give ma some advice, email me, thanx

  2. Betina Smith Says:

    I have a cory catfish which has been acting funny for a week and a half. Just laying around, I put him in a seperate tank and used internal and external parisite tabs. I just got some copper sulfate and hoping he will be o.k. I’m wondering if its too late to save him!

  3. Asso Secco Says:

    3-4 weeks ago my goldfish started to get some black spots (expecially on the fins’ borders, just 2 on the body) but it seems to be quite healty… it’s quite young so i think it’s just developing a “calico” appearence (before getting the spots ih was about 50% white and 50% red)… is that possible? how can I understand if my theory is correct?
    Thank you…

  4. Jane Says:

    Does your fish have any black around the face too? i do not have answers for you but my goldfish is doing the same thing. ???? puzzled

  5. chris Says:

    one of my goldfish has black patches on its face and fins. i thought it might be ammonia burn, but only ONE fish has it. wouldnt they both have it?

  6. Paul Leon Says:

    It seems that all the aquatic shops do not seem to know or even heard of ‘Black Spot’? i have affected fish not only the black spot, which having read the above can understand that the herron has infected my fish via the snail route, however i understand is harmless to the fish, but i am having fish die off recently since the weather has changed from cold to recent hot weather, resulting in a big ‘green’ water problem.
    Whilst it is catch 22 as i have only just been able to clear my pond to see the fish below and it has made me feel a little sick because not only black spot but all my fish have like a cotton wool fungus from their bodies and particularly the tails on the ‘shabunkings’ and some of the ‘shabunkings showing horrible horrible sores where body joins tail fin, just had 1 die this morning, if i had been able to see my fish earlier i would of been able to treat? by the way i have my water tested regularly and the water quality is ok every time. just bought an air pump now to improve oxygen levels. any advise to help me out? Paul.

  7. margaret P Says:

    I have an outside pond and two months ago one of my goldfish developed a 1/4 inch round black spot on its head near its gill. It kept itself to itself and I put Medifin in the pond. The fish seemed to pick up and swimming around but now it has developed large black marks on its head and other gill. I have again put Medifin in the pond. Could anyone tell me what this is and how to treat it. The fish it about 4 years old but only about four inches long. Margaret

  8. Zara Says:

    I just got a goldfish 3 days ago and after a day it started getting black bits on its tail. Then we bought a new fish of the same kind and it got one black spot on its fin and it seems to be getting worse on both fish.

    Is this black spot disease??? Help please 🙁


  9. Aly Says:

    Help! I just got a new goldfish last week, and so far things aren’t going too great. it has black spots and patches on its body, and is bleeding from the gills. It’s lethargic and its dorsal fin is usually lying flat. Any ideas what it could be?

  10. div Says:

    My goldfish has developed a black spot on its forehead and the fins have developed black edges. pls help 🙁

  11. Lizzy Says:

    I got my goldfish and within the first week it started developing black and a few white spots on one side. I also notice a white floaty tubing coming off its body sometimes. It has got worse and now has dark brown, black and some white spots in clumps on both sides,the fins and around the face. What should I do? EMAIL ME!!! 🙁 Also im not sure if it is eating!!! Is this black spot disease or something else??!!?? thanks

  12. Geoff Says:

    I have a large Gold Fish in a tank with 6 other smaller fish. In the last 2 days the large fish has been lying at the bottom of the tank as though it was trying to bite it’s own tail (curled up) and doesn’t seem to be able to swim straight. Best I can find it has TB, any thoughts?? What to do??

  13. james Says:

    I have 3’x15″ tank ,Iam keeping six medium sized gold fish ,2 nos black moly and 2 angel fish. Last two days the gold fishi started sitinh at the bottom of tank and was quite. some time eat one of two grains of fish food. but after onday the sick one died.
    next day other one died. now I seperated the sick one to seperate tand.My water is
    bore well. we also drink this water.I put one small grain of potacius after watching
    this .is it ok what type of sesease is this.pl advice

  14. pat Says:

    I got 3 small gold fish 2 red an white 1 all white red spot on head. about a week after getting them the white one started getting black spots, larger than ick dots. all over it,even on the red spot. one of the other fish just up an died. 2 days latter the one with black spots died, the last one has got a small black dot on tail fin I would like to save him… what should I do? what is it? I want to get more fish so how do I prevent this from happening again…thank you for any advice…..

  15. sarah Says:

    My goldfish has a black line on its lip like a moustache and black patches on its fins, he hasn’t been eating and has keep going up for air also he’s done an abnormally large poo that seems to be discoloured . Although ever since I’ve changed his water and put some water safe chemicals in, he’s been as happy as ever but I’m really worried about his lip :'( some help me please

  16. Sheila Says:

    I have a pond in my back yard with 34 goldfish and just this week I lost 6. I did not see any tiny black or white dots on them and the ones that died seemed to be moving slow and stay in one place for long periods not like they did before. One started to swim more on his side but other than that was eating and had no discolorization to fins. I now have him in a separate tank and he just stays on the bottom in one place. I also have 60 babies in a separate tank that was in the pond. I have cleaned the entire pond out and powersprayed it all down. How do i know what the 6 fish died from and prevent from happening again. One has black patch by her tail but not sure if it is ick. She swims around normal and eats fine. If anyone has any ideas please let me know I dont want to lose anymore. Thanks

  17. Toni Says:

    My big fantail goldfish have developed black spots on their fins and now going to the tail. What do I need to do?

  18. Toni Says:

    It is also on their gills…

  19. Rachael Says:

    Hi I got a goldfish from a fair and it is only moving around at night I don’t know if that is normal or not and It has got a black spot just behind his gill on a bit on his tail near the end

  20. Pam Says:

    I see a lot of questions and desperation on this site with no answers. I had 2 goldfish and a very small catfish. One of the goldfish just turned up dead. There was nothing apparent as to the reason he died. Two days later, the other goldfish has started with black spot. The catfish is fine. I have been doing a 50% water change about every 3rd day and have a Whisper filter in the tank. I have a 5.5 gallon tank. Email me with a possible solution please.

Leave a Reply