If he's not currently being quarantined, I would put him in a separate tank. This will lessen the chances of an infection spreading to your other fish.
I'm not familiar with ropefish, but swelling is may be due to several reasons such as:
- Digestive issues (such as constipation, or eating food that doesn't agree with them)
- Internal parasites
- Internal bacterial infections
- Salinity issues (such as the amount of salt in the tank being too high for them, or being moved from one tank to another with a different salinity)
- Inability of the internal organs to carry out their normal day-to-day activities
Since he was bloated when you first saw him, it's likely not a salinity issue. I would start with a small amount of Epsom salt to see it that helps.
Epsom salt (magnesium sulfate, or magnesium sulfate heptahydrate) has a fluid reducing, laxative effect (on people as well as fish). You can get it at supermarkets and pharmacies such as Walmart, Walgreens, CVS, Kroger, Publix, etc. (It's not available at petstores.) Get a brand that does not contain perfumes or dyes. A small container will cost about $1.
Put some tank water into a cup. Stir in 0.25 teaspoons Epsom salt per gallon of tank water. Once the salt is completely dissolved, pour the solution back into the tank.
Salts do not evaporate, so you don't need to add it again. To remove it from the tank, just do water changes. Or, if you have live plants, they'll simply use it as fertilizer.
(Note: Since I'm not familiar with ropefish, I'm unsure how much Epsom salt they can tolerate. However, a dosage of 0.25 teaspoons per gallon for 1-2 days is probably a safe amount for most fish. If you notice that that he's reacting adversely to it, do a water change to remove it.)
If the Epsom salt isn't effective, let us know. He may need either an antiparasitical or antibiotic medication. If that's the case, give us as many details about his condition (and appetite) as you can. This can help people determine if the issue is bacterial or parasitical.