I wouldn't use it in an aquarium. Use the water straight from the RO
unit and add something like RO
Right to remineralize it if you're using it in a FW
system. We had a post about this a while back. If I remember correctly, the softener replaces the calcium (or was it carbonate...memory's leaving me) ions with sodium ions. I think the end result to that debate was that this creates an unnatural environment for the fish and stresses them. Is there anything wrong with your water other than being very hard? If not, you could mix tap water with RO
water to achieve the needed hardness levels for the aquarium. If you're using it in a SW
tank, use straight RO
water with nothing added for top off.