I too THOUGHT that was a good skimmer, but I recently replace it with an ASM and my water quality WAY better than it was. I had it hanging on the outside of my sump as well. I will say this, it did well for the first couple of months, but after things start to grow on the inside, you can't clean it like most other skimmers and eventually you will see you will have to constantly adjust it so that it will skim, even then, it won't pull what most other will. Just my opinion.