If it is a few barnacles on a snail, I would assume they are not large, and I would not worry about them fouling up the tank. I actually had some luck with some tiny barnacles that came on some LR for a few months (if that is success). Basically they are filter feeders. If you are dosing the tank for any other filter feeders, so be it. If you are not, I am not sure it is worth doing just for those guys. I guess you can try, since there are various LR animals that are also filter feeders, but don't dose heavily.