Shopper Will this fit on my Aeris Ion AT 400 reg? How do you determine whether to get large or small?
RODRICK M Unknown to me whether it will fit that reg. Large or small goes by the size of your mouth. Adult males would go for large. Small females or children small. Unless you know that you have a small mouth I'd stick with large. Good luck.
MICHAEL B I am not sure about determining the size. I use the large, and I have shared my regulator several times and very few have found that it was to large. But, the mouthpiece should fit , it is a standard throat size. If you have not used this mouthpiece before, you will be pleasantly surprised by the fit and the ease of keeping the regulator in place.
ANA M I don;t know if it will fit your specific regulator, but I can tell you that it fits all 3 of mine (Scubapro, Beauchat and Aqualung).
Unless you have a really small mouth, I'd go for the large.