I second citric acid or plain bleach (with no scent or other additives) -- seems like many recommend 10:1 water:bleach solution but I tend to go heavier on the bleach than that.
I'd soak for a couple of hours, clean, rinse well, then let it dry.
If you're worried about encysted stage of any fish parasites, I can't say for sure how long any could possibly withstand any of this but disinfect (like with bleach) then dry for 24 hours is something I've seen suggested for QT tanks in between uses. *do your own research there if that's your concern