If you have carbon pads in your filters then any medications you are putting in the tanks will be removed by the carbon. You only add a carbon pad to remove medications from a tank after successfully treating the fish in it. Otherwise you don't need them at all.
Basically if you are adding medications to any tanks right now, then take out the carbon pads and replace with normal pads. DO NOT throw away the coarse pads when they look dirty, that "dirt" is your bacteria!!!! That's what's keeping your tanks cycled. If you remove them then your tanks will have to cycle all over again each time, which will cause ammonia spikes and invariably cause a few sick fish who can infect the rest. As well as the sick fish coming in in the shipments you're receiving.
Just wash the coarse pads when the filter flows begin to slow down a little. Don't wash them under a tap! The chlorine will kill the bacteria. Just get a bucket of used tankwater from one of the tanks when doing a water change, and just gently swish the pads in that. Don't absolutely scrub them, just get the big bits of debris off and put them back in the filter.
I am not familiar with the design of a biowheel as I have never seen them here. Bt if there is any sort of other media in the filter apart from the pads, such as Ceramic tubes or noodles or plastic balls ect, then yes you will also have a bacterial colony on those as well. You can also rinse a few handfuls of that in the tankwater as well if it gets really gunged up and is impeding the flow of the filter.
