Hagen have most of their manuals on-line -
this is what I've found for the Fluval 305.
Looking at that manual, they say the bottom basket should contain prefilter, carbon or zeo-carb (a mixture of zeolite and carbon).
This is what they mean by prefilter and
this is zeo-carb. They also make something called
ammonia remover which looks very similar to zeolite and is what you might have in your filter.
I do not like these ammonia removing media in filters. At the start, they remove the ammonia so that there is none left to stimulate bacterial growth. Then they get full and stop working and you don't have any bacteria to remove the ammonia so the level of that goes up in the tank. The good news is that yours will probably be full by now if you haven't changed them since you got the filter so they probably won't absorb ammonia or medication any more but will be a home for the filter bacteria which would have grown after the ammonia remover/zeolite got full.
Leave all the media in the filter for now. Hopefully the barb will recover, and treat if necessary with the media in there.
But long term I would think seriously about your media. I would change the bottom basket to that prefilter or maybe a coarse sponge, any make cut to fit the basket. In the top basket I would put a ceramic medium with a layer of filter wool on top, any make will do,
this is Fluval's ceramic medium. You didn't mention what is in the middle basket, but the best would be more ceramic medium.
You would need to change the media a bit at a time. At the most, do one basket at a time with at least a month between changes - six to eight weeks would be better.
As I've mentioned, zeo-carb and ammonia remover are not a good idea as they starve the bacteria and are not as good a home for bacteria as sponges or ceramics. Carbon also gets full and stops working - but it's only uses are to remove the brown that comes out of wood and removing medication once treatment has finished - but you need new carbon for that. Carbon is a hangover from decades ago but it is cheap which is why filter manufacturers insist on using it.