I would start all afresh and remember that it is all about cleanliness so this is going to mean keeping the
ammonia and
nitrite and
nitrate levels down so this is an uncycled tank and that is going to mean doing water changes to keep the ammonia levels and the other levels down while medicating. NO CARBON in the filters. It removes the medications. Put both the fish in the hospital tank and medicate and test for ammonia nitrite and nitrate every day and you probably have to do 50% water changes every other day or so which is going to complicate the medication schedule. Something like this:
Day one give according to box directions:
Day two give according to box directions:
Day three do a 50%
water change and give one and a half of the amount you would be giving to the fish this day.
Day four give according to box directions
Day five do a 50% water change and give one and a half the dose that you would be giving this day.
If you need to repeat for more than 5 days just keep going this way with a 50% water change every other day and give one and a half times the dose of medication on that day. You can give most of the meds for up to 14 days.
After 14 days put a carbon filter in the the filter and remove the medications and if they still need medications you will have to try another medication as the one has done all that it will. But let the carbon filter run for at least 3 days before trying another med. Discard the carbon filter before starting another medication.
Rose