One thing I've done to incorporate ceramic ring media into cartridge based HOB
filters is to string then together with fishing line. Perhaps a foot in length to start with. Leave a bit of slack between the rings to allow some wiggle room. Then drop it into any open space next to the cartridges. As you drop them in, use a weaving side to side pattern so that you use up as much available space in the housing. When it's time to do maintenance, simply lift the string of rings out of the filter housing.
I learned the hard way that dropping the individual media rings in the filter is a pain to deal with later. Thus, the fishing line.
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