I use some bonded fiber that comes in sheets and I cut to fit. It is blue on one side and white on the other. When I need carbon I add it in a media mesh bag. The fiber can be kept in place even when medication is in the tank and rinsed in tank water to be reused as long as possible. It does a fairly good job of holding together and if I need to add another piece it is easy to do.
I am medicating 2 tanks right now. (1 tank for mouth fungus and the other for gill spots) When the meds are finished I will replace the pad with a new one and a bag of charcoal for each compartment and in a couple of days I will take the carbon bags out and leave the pads in place.
Rose
