1st, ich is not always in your tank. If they are infected, the parasite was introduced into their tank somehow--either a new fish, they picked it up in the LFS prior to purchase (depending on how long you've had them), or even plants from an infected tank.
Clowns are scaleless, and you definitely should not use medications that are not labelled as safe for them. You can get ich medication that is safe for scaleless fish, but it's hard to find and costs a bit more.
Best reatment--1-2 tsp of salt (NaCl, not epsom salts) per gallon, added slowly to the tank (remove a few cups of tank water, dissolve the salt in there and add that back in, repeat every 3-4 hours til you've got the right ratio) and raise the temperature to about 83-84 over the next day.
This does 2 things--it shortens the life span of the parasite, because it's life cycle is temperature dependant. The salt will help the clowns, and will help kill the parasites (osmotic balance--this isn't high enough to kill them out right, but it does kill off some of them). Do frequent water changes and clean the substrate well each time. Keep the salt in the tank and the temp up for at least 10 days AFTER seeing the last crystal on the fish. Don't quit medicating immediately after, since the spores are still in there, waiting to hatch. You may need to increase surface agitation to up dissolved oxygen as well--warmer water holds less oxygen.
