im sure this gets asked a lot but I need to know what size diffuser I need to get ill be ordering off of aqualabtech, all their sliders say like 14/18mm, /14/14, I was also wondering what the first number n second meant so I know, now I did the "dime test" and I can fit a dime into the opening but it wont fully fall through so does that means it 18mm? I have a picture showing measurement of the opening, thanks for the help