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. _________ 75g reef with Nova Extreme 8 bulb HO T-5 lighting, refuge, Remora skimmer, DSB, Seio powerheads, Acro's, LPS corals, assorted zoo's & mushrooms, Yellow Tang, Maroon Clown, Blue Cleaner Goby , Cleaner, Fire and Sexy Shrimp, Coco Worm, T.Crocea Clam, Derasa Clam
120g FOWLR, 260w Orbit power compact lights, Remora Pro skimmer, 30g DIY Sump/Refuge, DSB,Seio Power heads, Powder Blue Tang, Purple Tang, Flag Fin Angel, False Eye Puffer, Copperbanded Butterfly and a Moorish Idol  | 
