TankGirl
Aquarium Advice Addict
Okay. Several things. You do need media in your filter. As mentioned above, the only thing that needs to be removed is carbon. I don't know the exact configuration of your media but if your sponge is just a sponge, with nothing packed inside it, then put it back - you need it! The sponge itself is not going to interfere with the fin rot treatment, but experiencing a cycle is going to make everything worse. It is only chemical filtration, in the form of zeolite pellets or charcoal, that will absorb the medication and remove it from the tank. If your filter does not contain charcoal then you do not need to do anything to your filter at all - I promise!
Also, I can't tell how large this filter is by the photo but I suspect you are underfiltered for the size of the tank and the bioload. I know your tank falls within the recommended tank size for this filter, but I personally do not pay a bit of attention to those recommendations, as they are almost always impractical. A small internal filter on a 20gallon tank is not going to do the trick, IMO, and this may be a contributing factor to what is causing fin rot for your betta.
Also, I can't tell how large this filter is by the photo but I suspect you are underfiltered for the size of the tank and the bioload. I know your tank falls within the recommended tank size for this filter, but I personally do not pay a bit of attention to those recommendations, as they are almost always impractical. A small internal filter on a 20gallon tank is not going to do the trick, IMO, and this may be a contributing factor to what is causing fin rot for your betta.