Hello - Advice needed

[Vicar of Debily]

Hi everyone. Introducing myself. I joined a couple of months ago, but my computer then crashed! All set up again.
I've been keeping barbs for just over a year now and in May upgraded to a 60 litre tank. We keep rosies, green tigers, odessas, cherries and a couple of midget sucker catfish to control the algae.
We added the cherry barbs just over a month ago and one of them appears to have fin rot. As he has been hiding in the castle it has been hard to tell, but he is coming out more this week and his fins are clamped, he is almost clinging to the gravel and doesn't appear to be eating. We have bought a treatment but it says to remove both carbon and zeolite filter media. Our filter uses both and we can't remove both without removing all 3 filter cartridges. I knew the carbon would have to be removed. Any advice about the zeolite? Would it also absorb the medication? We really want to help this little fella. I'd be really grateful for any advice on how to help the little one. Thanks.