Rockwool is cheap so buy it as roofing insulation as this is cheapest way to obtain it cheap enough to always be the medium. It is natural spun volcanic ash so is not bad for environment. One of the best water filters i think is a garden wall over grown with plants.
Even if never used it for plants and filled one with plastic one with poly fiber one with carbon one with coco nut husk and that would actually qualify as reverse osmosis.
I would stick rock wool lover top layer and make that as the filter.
https://www.gstore.com.au/wp-content...p_1__81309.jpg
eg: Let plant above water plant below. Use any plants to filter the water but aquatic plants with loads of water grow fastest eg indoor plants not full sun.