I buy RO
water and I add a small amount of Replenish to it. You only need to use a small amount. It replaces the minerals. A bottle lasts about a year and I do weekly water changes on my 75 & 45 gal
tanks. I buy it from Drs Foster & Smith or my lfs
. I don't think Petsmart or Petco carry it and it's cheaper online anyway.