You can use it to refill. Get some Prime, a water conditioner made by Seachem. Petsmart and Petco don't have it, you'll have to get it at a local fish store or online. It's the best conditioner you can find. After you drain the tank, you add Prime(per the bottles directions), and then fill the tank back up. Add enough prime to treat the entire tank(55 gal, so about a full capful), not just enough for the water you are putting back in(30%, 18 gallons, 1/3 capful). You can add prime directly to the tank.