In oscars it is usually not HITH, it is HLLE. The difference is that HITH is parasite based and is usually only found in discus and angels. HLLE is not parasite based and is usually caused by low quality food and water (especially in oscars). I have seen a couple cases of HLLE that were caused by carbon. The idea is that the carbon removes vital trace elements and micronutrients. It is almost always caused by low quality food and/or water in oscars.
What is the nitrate concentration? 'Good' is not an acceptable answer, we need a number.
What test kit are you using?
What is the nitrate concentration of your tap water?
What is your current water change schedule? 'Adequate' is not a good answer, how much and how often?
What are you feeding? Include everything you ever feed them, no matter how infrequent.
You only need to remove the carbon from the filter when using meds (not a bad idea to just never use it though except when you need to remove meds after treatment). You need to run your filters all the time, meds or not.
What type of filter do you have?