Ich Attack (100% organic) reccomends removing the carbon filter during treatment. With the filter I have, you cannot take the carbon out - it's one of those penn plax disposable filters where you replace the whole thing. What should I do?
Since bettas don't need filters as long as they are provided with clean water, you can place him in a hospital tank (2-3 gallon container perhaps - doesn't have to be pretty, just super clean) without a filter (with heater though of course) and do daily water changes. And I guess you would have to keep adding the same dose of the medication with each water change.
Put the fish in a different container to treat for the ich. I use rubbermaid or sterilite plastic storage bins--they make excellent hospital tanks because they are long and shallow and are safe to use with a heater. They are also extremely cheap--a 2-4 gallon bin is only about $3 at Target or Walmart. They are good to have around.