i have a 20 gallon tank setup for about 45 days now. The HOB filter was not keeping up so I bought a Marineland C220 canister. I have read it's much better to replace some of the filter media with Eheim Substrate pro. The issue is, I don't know which media in the filter I would replace. I don't remember the order of media but it has charcoal, ceramic beads and the plastic balls. Please help so I am sure to replace the correct media with the Eheim and Putin in the correct order in the filter.