To be honest it makes no odds whether u install it b4 or after it goes into the filter, however most i have seen is on the outlet, like ours.......
The advantage is that the UV steriliser is exposed to less "crap" and is only treating clean (in a visual sense) water, as its supposed to....
Im gonna charge my cam a take a photo.......
Here is where we got ours from - very competitive and very good - Vecton are as good as any....
We run the 8v on a 35ukgal but volumes are explained on the site. Its more to do with how fast the filter pumps and size of tank..... check ur filter box.