I believe it has to do with the size of the cup.1. for the classic outlet lily pipes, what's the difference between the P-1,P-2,P-4, and P-6? Amano's website says their all 13 mm but they're different prices. Is it the material they're made of or something?
Its just a different style. One of Amano's aquarium lines consists of metallic equipment. For your other questions, see if someone can help you over at suiso's forum,4. What the hell are those metalic looking ones for? there are 4 of them on the merchant site, 2 intake and 2 outlet (i think).