Want to give your top off water a little boost in the calcium department but hate mixing up kalkwasser and having to worry about pH spikes? Or is your salt mix just not adding as much calcium to the aquarium that you hoped for? A quick and relatively inexpensive remedy to both of these situations is to add a canister containing some type of calcium media to your reverse osmosis unit. The media of choice for this situation is aragonite, much like you would find being used in calcium reactors. You won’t get a huge boost of calcium with this method, but as you cycle water through the RO unit it will pick up some additional calcium that can give your tank just a little more to work with.