What’s causing these lumps on my fish?


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Fish expert Peter responds to a reader who’s fish has developed lumps…

Q) Is it possible to identify what these lumps are on two of my Black widow tetras, from the photos? The fish were purchased in July last year. I noticed the larger lump at the rear of one of the fish about a week ago – it seemed to appear very quickly but could have been developing for a while. There is now also a small lump on the throat of one of the others. They are eating well and seem otherwise healthy. These fish share their tank with three Calico mollies and five Cardinal tetras which were all added in March this year.


Peter replies: On the fish with the larger growth between the anal and caudal fin, the lump appears to have dark and white areas plus some small ‘bubbles’ (possibly vesicles) or cysts. The dark area may be the fish’s natural pigmentation, or it could be melanin pigment that has formed in response to disease. On the other Black widow tetra, I can see a small lump protruding from the chin area.

It’s not possible to make a diagnosis based on images alone, although I’m pretty sure they are not viral in origin, so not lymphocystis, or anything caused by the more common skin parasites, such as ich, velvet and suchlike. I also don’t think it’s bacterial, although we do occasionally see skin vesicles being produced as an unusual response to certain bacterial infections. I cannot rule out these lumps as being skin tumours, but it would be somewhat unusual to have two fish developing tumours at the same time, given these sorts of abnormal growths are, with a few exceptions, non-infectious. Ideally, one would examine these lumps closely and take some tissue samples for microscopy and other tests in order to reach a diagnosis.

So, with just the photos to go on, my best guess is some sort of microsporidial parasite infection or a Dermocystidium infection. Of these possibilities, I’m leaning towards Dermocystidium or something closely related to it. Dermocystidium is an enigmatic group of pathogens that shares features with the fungi and protozoa. There are 20+ species of Dermocystidium and some are known to affect ornamental fish, including Koi and various tetras (with several reports in Cardinal tetras) and South American cichlids such as Apistogramma. The fact that you recently introduced some Cardinals to the tank adds to my suspicion that Dermocystidium could be the culprit. If so, it’s possible that these new fish were healthy-looking carriers of Dermocystidium and passed the infection to your Black widows. I could be wrong, and in any case please don’t blame the shop where you bought the Cardinals, as the fish may have appeared completely healthy in the dealer’s tanks.

In cases where Dermocystidium has affected tetras and cichlids, the infections typically manifest as clear bubbles or cysts.  Over time, you may see a creamy-white tubular shape develop within each bubble which looks just a like a creamy worm, which would almost certainly confirm that the diagnosis. This tubular structure contains many thousands of tiny infective spores.

Unfortunately, Dermocystidium infections are untreatable. On the plus side, it is possible that the species that’s affected your Black widows is limited to tetras and hence won’t affect your other types of fish. But we cannot be sure, as we still have a lot to learn about Dermocystidium species and their fish host ranges.

I think at this stage, you should monitor all your fish and avoid buying any new stock until this current problem has resolved. I see no point in using medications, given we aren’t confident as to what we are dealing with and, if it is a microsporidial infection or Dermocystidium, neither is treatable anyway. If you have a spare tank, it might be worth isolating the two affected fish, although chances are that any infection would already have spread by now.