I have a CS6-1 in a 10 gal sump w/ a 20 gal display tank, I was going to get the 5-2 but I was able to fit the 6-1 in my sump it is crowded but I am glad I did it....I was thinking about the 6-2 for the larger pump, but there was not enough room....I always thought I had clear water, but my water now is CRYSTAL clear...it made a big difference, I had HOB Remora with a Maxijett1200, than I added a HOB overflow, and sump, and now its MUCH improved