Yeah, three years sounds like a long time for 13". I'm glad you're doing something to boost their growth.
Everywhere you go, people will tell you that violet gobies are blind, or almost blind. I don't think this is true. Mine does everything in his power to keep me out of his territory. If I'm walking or sitting in front of the tank, he'll come out and display his fins at me, even standing up on his fused pelvic fins. Do any of yours do this? I've heard that males get more aggressive, so I think mine is a male. In that picture it looks like that friendly "smile" might actually be a territorial display.
I finally figured out how to get good pictures so I might have a couple on here later tonight.