The only way to do this properly and safely, Denise, is to get rid of the goldfish, sterilize everything, and start the necessary cycle with a fishless approach using store bought ammonia. You could try to use a 'quick start' product that is effective over time but you'll still need an ammonia source to feed the bacteria (other than fish), or obtain a seeded sponge from the your discus source when you get the fish.