Did you treat the main tank as well? If not, 4 weeks with out fish is not enough time to kill ich. You need 8-12 weeks, the longer the better with the tank temp 82-84F. Your neon goby may have also had the parasite. Any stress to fish can bring it out. Try using a rubbermaid tub as a QT tank. or just run 2 10g tanks. The tanks are probably cheaper than the tubs. $9 at petsmart.