Saying your water tests are ok doesn't help us very much.
Water should test 0 for ammonia and nitrites all day every day.Water changes of 20 to 25 percent each week using a dechlorinator that detoxifys chlorine,chloramines,and AMMONIA for new water before it is added to the tank is also necessary.
Filter should be mature, (five to six weeks) old with healthy population of bacteria.
Product Quik Cure should be used for three consecutive days at half dose for sensitive fish like tetras and catfish with 25 percent water change on fourth day. If Ich is still present,repeat the treatment.
Don't feed the fish during treatment. turn lights off during treatment. Slowly raise temp one degree each day to 82 degrees. Remove carbon from the filter during treatment. Replace carbon with new carbon after treatment. Don't add any new fish to tank during treatment. Don't use nets buckets etc that are used for sick tank in other tanks.
This info should help you.