I really don't know about barnacles, but I do see them when I'm fishing in S Florida. They anchor on posts and boats etc. But I see them in the intracoastal which isn't the cleanest water way.
I have one in a 6 gallon eclipse FWLRO. It came as a hitchhiker and I've had it for 2 years. When I first got the rock it was as small as a grain of sand now its 1"x.5".