[paulamrein]

Well, I am sure you have heard this before, there isn't a tang out there that will be able to survive in anything less 75g. The flame angel needs at least 30g. It's not so much the amount of water, it's the measurements. They need a lot of room to swim. The clown and damsel are fine because they don't need that much to move around. Having said that, put any fish in there you want. It's no one's place to tell you what you can/can't keep. With the remaining fish, you have to take them out of the tank and treat them leaving your main system without fish, all inverts can stay. There are hundreds of threads dealing with this subject. Some of the top threads on this forum is a great place to start. What is suspected to be your problem is either ich, or velvet. You MUST treat the fish, and you must treat it in a separate tank. Treatment will take on average anywhere from 4 weeks to 8 weeks. Depending on method used. While you are treating the fish in the QT (quarantine tank) the main tank will also rid of the parasites because the parasite has no host to complete it's life cycle. These parasites need fish to complete it, that's why it is ok to leave any non fish animals in.
After the 6 to 8 weeks are over, (I would suggest 8 just to make sure) you can put the fish back in the tank and probably never worry about parasites. AS LONG as you qt everything else going into your tank for another 8 week period. Everything means Everything.. things that eat or don't. rock, sand, snails, anything wet. Even water if it has had fish swimming in it. It's not what you wanted to hear, I didn't when I heard it. But, the 2 months your tank will be empty is a drop in the bucket of what all the benifits come out of it.
Again, don't let anyone tell you what you can or can't put in your tank, all I ask is that you research from more than 2 or 3 sources about anything that goes into your tank before you buy it. The LFS will sell you anything. There are more opinions on this hobby than I have heard about any subject. It doesn't help you, doesn't help the fish, all the uninformed person does is help perpetuate the lies that are in the trade. Ask any other questions you want and read, read, read. And when you are sure you know the fish you want to buy, ask atleast 3 people that own the fish. Get into some local clubs, or a forum you can get to know people in. Knowledge is the ultimate life and money saver.