could someone please explain how joint sizes work to me? i am going to buy this when it gets back in stock: BioHazard 7mm - Triple Tree - 5 arm Percolator Bong - Dutch - Grasscity.com but i need to know if i need to get the 14.5mm or the 18.8mm octopus: Pre-Cooler Octopus - 18.8 - Dutch - Grasscity.com vs. Pre-Cooler Octopus - 14.5 - Dutch - Grasscity.com which do i need? thanks a ton, bpuff.