Reverse-Osmosis (RO) household water filtration systems are a set of semi-permeable membranes (to me the systems often look like a couple of those screw-on automobile oil filters visually) that attempt to trap virtually all the solutes (thinking of water as the "solvent", the solutes are all the "other stuff" that's distributed evenly in the water ("dissolved") like minerals and metals on one side of the membrane, while allowing only solvent (the pure water) through to the other side of the membrane. RO units do this via the water pressure itself pushing the source tap water through the carefully designed membrane. A LOT of water gets wasted. I believe only about 15% or so of the water comes though to be the water you use. You then take this "water that has nothing in it" (which of course makes it something fish would not be evolved to deal with at all!) and mix it back in some ratio with the tap water that has good calcium and iron and other minerals in it, such that you've now got water that has some degree less hardness to it.
Using RO mixed water is probably an easier choice in those UK areas where water is still un-metered (free?) but can be a difficult choice in some USA localities where water can be quite expensive, given the great loss of water caused by the process. Figuring out the mix of RO and tap that you need to get your water back to the hardness level you want can be helped by the use of an inexpensive electronic TDS (total dissolved solids) meter I believe. Note that my description of an RO unit is an oversimplification. In reality I believe they have to have carbon layers before and after the membranes.. I believe carbon is used not only to remove organics but also chlorine/chloramine because it would damage the material the membranes are made of. I believe the "business end" of the RO units (membrane, carbon etc.) is something you must buy and replace periodically.
~~waterdrop~~
