I'm probably not the most reliable person to get advice from as I've only been in the hobby for the past few months - but! I did have a pretty bad ick outbreak in my 20 gallon with three platies. I managed to beat it in under a week thanks to a bunch of advice from pros. Of course, if this is a past tense kind of thing, just disregard this entire post.
First: I would get a good ich treatment. Go for API Super Ick Cure. This stuff is bomb. A blue bomb. It'll turn your plastic decor green. Activated carbon will turn it back to its original color though.
Second: Turn up the heat. Make sure you check how warm you can get the water without stressing our your fish more.
Third: Please dont take my word for this one, but I added salt to the water.
I am by no means at all a pro with any of this stuff, but from experience these three things got on top of ick pretty quick. Still, I would do some extra research, I could for all anyone knows be a total moron.
Happy fishkeeping!