20 Gallon Tank Lumps/cysts/nodules on platies. Lymphocystis?


Hi everyone, I hope all your finned feel great and do well!

I haven't been here for a while, but I recently ran into some strange lumps appearing on some of my platies in both fish tanks. So far I only had one death that seemed to be directly related to this kind of lump, but I am still not sure what exactly I am looking at.

So the first subject was a female platy (let's call her Platy 1) in my 20gal female-only platy tank. This tank had no changes to inhabitants in more than a year (no new fish added, I was only removing fish that passed from time to time). 20 gal, 0/0/40 (a little high on nitrates), some overcrowding (maybe 15-20 platies), 76F, 2 filters (one HOB and one small "backup" sponge), air stone, only artificial decorations and sand at the bottom; water changes once every 7-10 days (~25%) with Seachem Prime, daily moderate feeding mostly with Hikari or NorthFin pellets. No major outbreaks or multiple deaths for the duration of this tank (~3 years from the start).

Some time ago (we are talking months here, probably before last Christmas) I started noticing some reddish discoloration on the body of one or two platies - one at the tail area and another one (this Platy 1) right on the body. My first though was some fin rot, so as a preventative measure I started adding some Seachem ParaGuard alternating it with StressGuard for a week or so. It didn't seem to affect this lump at all and it kept growing. It didn't visibly impact the fish at all, it was still playful, quick to come for food etc.

Roughly 2-3 weeks ago this lump already reached a very impressive size and started affecting her ability to move quickly (probably due to extra resistance to water). I tried spotting good pictures with my phone, but failed terribly at this as she was fast, but I hope you'll get the idea.

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Then roughly 2 weeks ago she started getting worse very quickly, spending more time in secluded areas of the tank and, unfortunately, within just a few days it was over for her.

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I was trying to see if there were possibly any parasites (?) in this cyst, but I haven't seen any even under magnification.

Now I started noticing that lumps on another platy started growing really fast lately (on her back close to her fin) and it gets me quite sad - I think she's gorgeous! It's still much smaller than on Platy 1 in the end, but other fish started attacking her from time to time, nipping on these wounds and I feel bad for her.

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I googled a lot of stuff and basically it came down to either carp pox (probably no, as it's specific to carps/goldfish/koi) or lymphocystis which I am also unsure about (some sources say it mostly affects cichlids and bigger fish and I haven't seen mentions of it in smaller livebearers).

Any clue what I may be looking at? I will keep adding StressGuard / ParaGuard from time to time, but it doesn't really seem to help. I would love to quarantine her (at least to make her safe from attacks), but I have no quarantine tank to safely put her to now :-(

Any thoughts/ideas? Thank you very much!

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What is the water volume of the tank? 20gal
How long has the tank been running? 3 years
Does it have a filter? HOB + sponge
Does it have a heater? Yes
What is the water temperature? 76F
What is the entire stocking of this tank? (Please list all fish and inverts.) 15-20 female platies


How often do you change the water? once every 7-10 days
How much of the water do you change? 25%
What do you use to treat your water? Seachem Prime
Do you vacuum the substrate or just the water? Vacuum over sand + poke it at least once a month to prevent trapped gases to accumulate

*Parameters - Very Important

Did you cycle your tank before adding fish? Yes
What do you use to test the water? API drop test
What are your parameters? We need to know the exact numbers, not just “fine” or “safe”.
Ammonia: 0
Nitrite: 0
Nitrate: 40 or more
pH: not tested recently, I have relatively low pH soft water here, prob close to 7.0 or less


How often do you feed your fish? Daily before going to sleep
How much do you feed your fish? Try moderate servings
What brand of food do you feed your fish? Hikari or NorthFin pellets
Do you feed frozen or freeze-dried foods? Not recently

Illness & Symptoms

How long have you had this fish? 3 years; all fish were born at home, descendants of 2-3 females I had 3-4 years ago (no new fish added recently)
How long ago did you first notice these symptoms? 6+ months ago
In a few words, can you explain the symptoms? Cysts/lumps on the body that slowly grow over time on several fish
Have you started any treatment for the illness? Tried StressGuard / ParaGuard with no visible impact
Was your fish physically ill or injured upon purchase? No
How has its behavior and appearance changed, if at all? Not much until very close to passing


It may be lymphocystis based on the cauliflower appearance and occurrence in multiple fish. Lymphocystis is not very contagious, however, you don't want to allow other fish to nip at the wart.

It can be a tumor, although occurrence in multiple fish is a bit suspicious. Interestingly, in both cases it grows out of a black patch, which is consistent with those being prone to cancer. If the development of this color form involves cancer inducing genes, then it is possible that multiple fish experience it. In a strain of swordtails that has a lot of black patches and spots, I see some fin erosion on the black fins of some of them. However, mine do not develop anything with the cauliflower appearance.
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