I have the 18" on the back of my 46g. Granted it is not a whole lot of extra water you can rotate through your tank like Bangguy has, but I have noticed I am able to controll my nitrates and other tank levels much easier than I was without it. I figure a little is better than none. But since the light for the fuge is on the back parts of my tank are lit 24/7. I had to add a backing to stop most og the light from bleeding through to controll my alge blooms.
HTH
Milt