Any chance of another picture, preferably with the fish in focus?
I am going with intestinal worms that are lost in the fish. Instead of living in the intestine, the worms have become lodged in the abdomen and are living there.
Praziquantel is available from most pet shops either as a gill fluke treatment for fish or a tapeworm treatment for cats & dogs, and it should do the job. If it doesn't then try Levamisole or Flubendazole.
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Before you treat the tank, do the following things.
Work out the volume of water in the tank:
measure length x width x height in cm.
divide by 1000.
= volume in litres.
When you measure the height, measure from the top of the substrate to the top of the water level.
There is a calculator/ converter in the "FishForum.net Calculator" under "Useful Links" at the bottom of this page that will let you convert litres to gallons if you need it.
Remove carbon from the filter before treating or it will adsorb the medication and stop it working.
Increase aeration/ surface turbulence to maximise oxygen levels in the water.
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You can use Praziquantel to treat tapeworm and gill flukes (the things in the fish are generally lost tapeworm or another type of parasitic worm). And Levamisole to treat thread/ round worms. If you can't find these medications, look for Flubendazole, which treats both lots of worms.
You treat the fish once a week for 4 weeks. The first treatment will kill any worms in the fish. The second, third and forth treatments kill any baby worms that hatch from eggs inside the fish's digestive tract.
Treat every fish tank in the house at the same time.
You do a 75% water change and complete gravel clean 24-48 hours after treatment.
Clean the filter 24 hours after treatment too.
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.
The water changes will remove most of the medication so you don't overdose the fish the next time you treat them. The gravel cleaning will suck out any worms and eggs that have been expelled by the fish. Repeating the treatment for 3-4 doses at weekly intervals will kill any worms that hatch from eggs. At the end of the treatment you will have healthier fish.