You can get a surface skimmer for fresh water tanks (I guess you can use them on SW too). Basically, it works with a canister filter and, as the name implies, skims water from the surface. They can be adjusted to take as much or as little water from the surface as needed. I use them since my tap water tends to form a light film when it stands so my tanks will develop a light film on the top. The surface skimmers keep it skimmed off and clear. I have to use airstones in the other tanks to keep the film broken up.