How many gallons is your tank? Unless its 15 gallons you most definitely need more snails. A general rule of thumb (at least one that has worked for me and plenty of other people) is one snail per 5 gallons. I have a 40g so I have 8 snails and I honestly sometimes feel like I need a few more.
What kind of goby do you have? I also have 2 clowns and I plan on getting a six line and a goby. I'm wondering how your goby and six line are towards each other because I can't decide what kind of wrasse and goby would work well with my stock.