You should wash off your filter media in the dirty tank water when you are doing water changes. As for cleaning decorations, I would leave them be unless you have something weird growing on them because they help hold good bacteria.
If it is the filter itself then I would take out the media and clean the filter with some hot water and then put the media back in to keep your cycle going (If you're cycling) If you aren't I would still wash your filter with hot water and just put in a new filter pad.
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