Not long after I set up my reef tank, I noticed the water surface began to look a little scummy. I went to the hardware store and bought some 1" PVC and a cap. I also bought a nylon screw. Back at home, I cut the PVC just long enough to fit well on the suction tube of a small HOB tank filter. I jammed the cap on the PVC (didn't glue it), drilled and tapped a hole for the nylon screw and slid the assembly over the suction tube of the filter. I tightened the screw against the suction tube to hold the assembly in place... instant surface skimmer! The screw is only finger tight. That allows me to adjust the height of the PVC so that the filter suction removes the scum from the water surface without sucking too much air. I keep it just low enough to not suck any air. Then when the water evaporates and the level drops, it sucks air and I know its time to top off the tank with fresh water. I never have to worry about my salinity because this helps me maintain the correct amount of fresh water additions.

For a while this doubled as a somewhat inefficient protein skimmer also. When it sucks air (which it normally does every day, sometime before I get up in the morning), the impeller on the pump makes millions of tiny bubbles which removes some of the protein solids and deposits them on the filter media. It worked great for the first few months until I finally got a real protein skimmer.

I still use it as a surface skimmer... keeps my water surface crystal clear! It would work on a fresh water tank the same way. I'll probably use it until I get my HOB overflow and sump set up. Within just three minutes of installing this in my tank the water surface was clear.