What is the dimensions of your tank?
These 'black box' lights only come in one size. They will easily cover 2 ft of tank area.
Another question... I have a marineland filter right now and I don't like it al all, I always have to clean it out and it seems algea is always on and all around it, Is there another better option I have, I've heard that external filters work quite well, though I only have a 30 gal, what do u think???