Without knowing what disease your dealing with, it is not possible to be of too much help. Posting a good picture of the fish with the disease can help a lot. However, there are several specific parasitic diseases you should look for. I have posted this before, but it's worth repeating here.
DaveK's Standard Lecture #3 – Common parasitic diseases
Look for these three major parasitic diseases that clownfish, and for that matter most other fish tend to get a lot.
1. Ick, officially known as Cryptocaryon irritans - look for small white dots that seem to fall off in a day or two, sometimes leaving behind small circular wounds. Treatment is by hyposalinity or copper.
2. Velvet, or gold dust disease, officially known as Amyloodinium occellatun. Looks like the fish is covered in a coating of gold dust, and the fish is often going around scratching. Treatment is by copper.
3. Clownfish disease, officially known as Brooklynella hostilis. Looks like gray irregular patches. Treatment is Formalin.
All treatments must be done in a quarantine tank.
As a corollary, or possibly "corallary" (grin), I will add the following -
Never, ever medicate a reef system to treat a fish disease.
No matter what the LFS says, no matter what it says on the bottle or package, no matter what someone else may tell you, in my opinion there are no "reef safe" treatments that are effective against fish diseases.
Always treat in a quarantine tank. Although, if you are treating a different disease, you might use a different medication.
Just so everyone is clear on this, I shall repeat this advice -
Never, ever medicate a reef system to treat a fish disease.
---End of the Lecture ---
Some additional thoughts -
If your fish have SW ich, then hyposalinity is the recommended treatment, but if it's something else, then it must be treated differently.
Unless you want to devote the tank to a single fish or get a massive tank, I recommend you return the lionfish to your LFS. They grow fast and get big. 12 inches isn't unusual, and it depends somewhat upon the species you have. The problem is that they can consume fish almost as big as they are. So if you want to keep something with it, you need to have any tank mates about 1 1/2 times the size of the lionfish. This means you need a massive tank if you want to have more than just a lionfish.
The feeder fish you feed to a lionfish should be SW fish of some sort. Using guppies, rosy reds, goldfish or other FW fish do not provide a complete diet. The good news is that with care, and some work, you can train the fish to accept frozen foods such as silversides.
The one other downside to lionfish is that they are venomous, which means keeping a constant eye on them when ever your working on the tank. You always need to know where the fish is,
