Platy with swelling near tail fin

Hello all - having a problem with my one and only remaining male platy. He has developed a large bulbous swelling near his tail which looks serious but try as I might I can't identify what it could be. He is still eating well though but spends a lot of time at the top of the tank.
Water parameters are all ok:
Ammonia 0
Nitrite 0
Nitrate 25
Ph 7.6

I have attached not great photos - any ideas what it is and a possible treatment.

How long have you had the fish for?
Did it always have the patching on the body (it looks like scales are missing)?

The fish has clamped fins and either a protozoan or bacterial infection.

Depending on what else is in the tank, you can try salt or use a broad spectrum fish medication that treats bacteria, fungus and protozoan infections.

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You can add rock salt (often sold as aquarium salt), sea salt or swimming pool salt to the aquarium at the dose rate of 1 heaped tablespoon per 20 litres of water. If there is no improvement after 48 hours you can double that dose rate so there is 2 heaped tablespoons of salt per 20 litres.

If you only have livebearers (guppies, platies, swordtails, mollies), goldfish or rainbowfish in the tank you can double that dose rate, so you would add 2 heaped tablespoons per 20 litres and if there is no improvement after 48 hours, then increase it so there is a total of 4 heaped tablespoons of salt per 20 litres.

Keep the salt level like this for at least 2 weeks but no longer than 4 weeks otherwise kidney damage can occur. Kidney damage is more likely to occur in fish from soft water (tetras, Corydoras, angelfish, gouramis, loaches) that are exposed to high levels of salt for an extended period of time, and is not an issue with livebearers, rainbowfish or other salt tolerant species.

The salt will not affect the beneficial filter bacteria but the higher dose rate will affect some plants. The lower dose rate will not affect plants.

After you use salt and the fish have recovered, you do a 10% water change each day for a week. Then do a 20% water change each day for a week. Then you can do bigger water changes after that.

Thanks for your response Colin. I have danios, neons and a pleco as tank mates. The pleco has been suffering with (I think) a parasitic infection which I've been treating with every anti parasite treatment I can lay my hands on,but with little success - he still has thin white stringy poo and I have recently noticed white patches appearing on him which look quite shiny in the right light. The tank is due for a water change on Friday following a second treatment of API General Cure after which I'll try an anti bacterial treatment such as esha 2000. The tank has also been treated with aquarium salt at 3g/ltr but has reduced to about 1 g/ltr now due to water changes. I'm not sure about using salt with a Bristlenose Pleco though.
I don't recall the platy having missing scales but it does look like some are gone, also the scales by the swelling are stood up like dropsy. All of this appears to have happened over the last couple of days.

Do not use medications unless you know what the problem is. Most fish health issues are caused by poor water quality and adding chemicals (medications) to the tank only makes the problem worse.

The following link has information about what to do if your fish get sick. It's long and boring but worth knowing. I recommend printing it out and reading it in bed to help fall asleep.
http://www.fishforums.net/threads/what-to-do-if-your-fish-gets-sick.450268/#post-3804819

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Do not use anti-biotics on fish unless they have a known bacterial infection that has not responded to normal fish medications.

Improper use or misuse of anti-biotics can lead to drug resistant bacteria that can kill people, animals, birds, fish and reptiles.

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Cream, white or grey patches on fish is excess mucous caused by chemicals, poor water quality or protozoan infections.

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Fish do a stringy white poop for several reasons.
1) internal bacterial infection causes the fish to stop eating, swell up like a balloon, breath heavily at surface or near a filter outlet, do stringy white poop, and die within 24-48 hours of showing these symptoms. This cannot normally be cured because massive internal organ failure has already occurred.

2) internal protozoan infection cause the fish to lose weight rapidly (over a week or two), fish continues to eat and swim around but not as much as normal, does stringy white poop. If not treated the fish dies a week or so after these symptoms appear. Metronidazole normally works well for this.
There is a medication (API General Cure) that contains Praziquantel and Metronidazole that can be used for internal protozoan infections.

It's interesting that API and the Californian government have listed Metronidazole as a carcinogen. That's a concern considering it was widely used to treat intestinal infections in people.
Anyway, handle with care, don't inhale the medication, and wash hands with soapy water after treating the fish or working in the tank.

3) intestinal worms like tapeworm and threadworms cause the fish to lose weight, continue eating and swimming normally, do a stringy white poop. Fish can do this for months and not be too badly affected. In some cases, fish with bad worm infestation will actually gain weight and get fat and look like a pregnant guppy. This is due to the huge number of worms inside the fish.

If the fish are still eating well, then worms is the most likely cause.

You can use Praziquantel to treat tapeworm and gill flukes. And Levamisole to treat thread/ round worms.

Remove carbon from filters before treatment and increase aeration/ surface turbulence to maximise oxygen levels in the water.

You treat the fish once a week for 3-4 weeks. The first treatment will kill any worms in the fish. The second and third treatments kill any baby worms that hatch from eggs inside the fish's digestive tract.
You do a 75% water change and complete gravel clean 24-48 hours after treatment.

Treat every fish tank in the house at the same time.

Do not use the 2 medications together. If you want to treat both medications in a short space of time, use Praziquantel on day one. Do a 75% water change and gravel clean the substrate on day 2 & 3. Treat the tank with Levamisole on day 4 and do a 75% water change and gravel clean on day 5, 6 & 7 and then start with Praziquantel again on day 8.

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All freshwater fish can tolerate the low level of salt (2 heaped tablespoons of salt per 20 litres of water).

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Take pictures of the fish and check them on your computer before posting them on here so we can see what's wrong. Make sure the pictures clearly show the issue.

Check your water quality for ammonia, nitrite, nitrate & pH.

Do 75% water changes and gravel clean the substrate each day for a week and see if that helps.

Stop adding chemicals until we know what is going on.

Hello again Colin and thank you for your reply. You have hit the nail on the head - identifying what the cause of the disease is. It would appear that this isn't as easy as it sounds. In the case of my platy with the swollen body near the tail fin shown in the photos I have been unable to find out what is causing it, in fact I cannot find any reference to this type of swelling in any publication. There are several online blogs referencing similar swellings but nothing actually identifying the cause or any successful treatments. As previously mentioned I would add a little salt but I have a Bristlenose Pleco which I don't think will tolerate salt well.
With regard to the Bristlenose Pleco with the stringy poo - I have been advised by three separate fish shops that the likely cause would be internal parasites which I have treated over the months with a variety of internal parasite related medications. The trouble is that none appear to have been successful even after follow up treatments. As you know the term "internal parasites" covers many different types of parasite each with it's own specific medication - I cannot understand how anyone can identify which one is causing the problem without treating with random medications until the right one is tried. I have been advised that identification of the ailment can be accurately made once the fish has died - great help! Regrettably unlike the U.S. and possibly yourselves we in the UK have to make do with low dose medications and no antibiotics, we cannot even purchase decent treatments from abroad. Anyway enough of my rant - have you any idea what ails my platy and pleco? and what should my next course of action be?

The platy appears to have a protozoan infection and maybe a bacterial infection too.
The fish with the stringy white poop probably has intestinal worms. Most fish have intestinal worms.
Fish with cream, white or grey patches on their body have a protozoan infection or are in really poor water quality.

Bristlenose catfish can tolerate 2 heaped tablespoons of salt per 20 litres of water for 2 weeks.

I would add salt and treat the fish with Praziquantel. After the salt treatment I would treat with Levamisole.
Directions for using Levamisole and Praziquantel together are in post #4.

Thanks for the advice Colin - I treated the tank with esha NDX (levamisole hydrochloride) three weeks ago and API General Cure (Praziquantel and Metronidazole) two weeks ago with a follow up dose last week. All medications were as per the manufacturers instructions with the filter carbon pads removed and followed by 25% water changes. As a maintenance procedure I always do a 25% water change weekly together with a gravel vacuum. I added salt a month ago to a ratio of 3g/ltr but this has now decreased to 0.8g/ltr due to water changes.
I'm intending to do a water change tomorrow and increase the salt content inline with your advice, this will bring the salt to 4g/ltr (42grams=1 tablespoon) - will my Bristlenose Pleco survive this? I will also redose with esha GDX (Praziquantel) as per instructions and follow this with esha NDX (Levamisole). At some point I will have to address the possibility that it may also be a bacterial problem but I'll try the praziquantel and levamisole first.
Any further advice would be most welcome.

You will know very quickly if your pleco can handle the salt. Mine made it quite clear to me that he doesn’t do well with it at large doses. He came out almost immediately in the middle of the day and started swimming like he was going nuts (if he could, he would have yelled at me). However, after I changed half the water and left half the dose in the tank, he was ok. Just watch him.

Deanasue said:

You will know very quickly if your pleco can handle the salt. Mine made it quite clear to me that he doesn’t do well with it at large doses. He came out almost immediately in the middle of the day and started swimming like he was going nuts (if he could, he would have yelled at me). However, after I changed half the water and left half the dose in the tank, he was ok. Just watch him.

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OK thanks, I'll keep an eye on him