Well, you'll hear a lot "you didn't QT your new fish". I know I did when I didn't QT specimen.
Just remember next time, QT them before joining them into your tank.
Do you have any inverts in your tank? Because if not, then you may be able to hyposalinate your 20G long. Your live rock will be just rock after the treatment. And your live sand will just be sand as well. But guess what, after the whole treatment, buy a small live rock to seed your existing rocks and sand and, voila, they're called live rocks again.
If you have inverts in your 20G long, you will have to buy a smaller QT tank to treat your fish.
Ich will weaken your fish. Don't be misled whenever you see those spots disappear. that means those ich dropped on the sand and will spawn thousands more of them. So you'll notice that in the morning, no spots, by 6PM, plenty of spots.
Read up on c. irritans. There are great articles over the internet on this. Good luck!
We live and we learn