There are easier ways. They say no water changes, but, imo, you can ignore that. The most important consideration in medicating is making sure the medications gets to the patient. In water dosed medications, they get to the patient simply by being in the water. So there are two key factors in the potential success of the treatment- adequate dose and duration of the treatment. As long as what you do does not affect these things, the level of success should be pretty constant.
In plainer English- you can do a water change as long as when you are done the amount of medication in the water is basically unchanged. So, all you must do is to replace any medication removed. If you do a 50% water change, you have removed 1/2 the medication so you will add a 1/2 dose back into the tank with the new water.
This becomes much simpler when the needed water change coincides with dosing. Some meds you may dose daily, some meds you may dose every other or every third day, some just once. If you change water on a day you are supposed to dose, all you have to do is change the water first and then administer the medication.
With medications not dosed daily, you may need to fudge a bit. For example, if a medication is done in one single dose and you wait some number of days to change the water at the end of the period, you do not want to add back the same amount of the medication if you do a water change on day 2 vs day 4. And this is where it gets less then scientific. You will have to use your best judgement. Do you want to add back a bit less medication to compensate for the time factor or not.
While the last example clearly is not precise, it is likely much less of an issue than the effects of ammonia on a sick fish because one does not change the water. One might argue that it is better to try to use a cycled tank and follow the dosing instructions to the letter. But this may not work if a med is also destroying the bio-filter in the H tank.
I have always worked with uncycled H tanks and changed water and compensated with additional dosing to restore the medication levels. But that is me. You will have to decide what you feel will work best for you in light of the specific situation. Bear in mind that when we start treating in an H tank, the first medication we try may not work. So even if it was filter safe, the next medication we need to administer may not be.
Finally, consider the cases where you fail and you are not quite sure what the disease/illness was. Good practices indicate you need to sterilize everything in the tank, including the filter and filter media. So in such a case you must kill the filter bacteria anyway.
