Goldfish tail fin rot help

Just lost 4 goldfish to what appeared to be tail fin rot. Tried to save them by using Malachite Green but was unsuccessful, lost three in one day and the fourth about a week later. So now i have an empty 10 gal tank and i want to get some more fish for it. I was wondering if anyone new the proper way to go about cleaning my old "death tank" so it is ready and safe for some new fish? What should i do and how long should i wait until introducing new goldfish? Any tips on how to prevent and/or better treat tail fin rot in the future would also be great. I love my fish and killing them breaks my heart i just want to have my healthy, happy tank back again!!!