Why my goldfishes are infected with fungus again and again?

I have started to pet fish from one month. Kept it in 12cm diameter bowl.. have 3 fish.. 1 gold and 2 red cap.. at first they were infected with paracites.. we cured from those..then fungus on their fins.. I have a normal filter and cleans the bowl and full water change in 6 days.. treated them with salt too.. have tried malachite green with one fish..and she died.. please suggest.. how do I help my 3 remaining fish infected with fungus.. it's very minimal and have seen before 2 days only.