My water wasn't that crystal clear and I was at a loss as to why for a while.. everything tested perfect, had filter socks and changed them every 3 days like clockwork, and an overflow filter in the sump as well.. macro algae, protein skimmer, gfo and carbon... but still dirty looking water.. now the tank was only like 2 months old and only had a few fish and rock in there, so I bought a 145$ hang on the back as a temporary lets see how it does UV sterilizer.. let it run over night left for work( didnt even look at the tank as I left) abd came home to "what the heck" all my water is gone!... it was so stupid clear it didnt even look like there was water in there.. I even to a picture to show a friend.. so now every week from about 8pm to 8am i run it and its been perfect ever since