There are now freshwater skimmers available, how effective they are I'm not sure as I haven't used one. I have used the salt-water versions though. They didn't used to work in freshwater as the water itself needed to have some salt content, if you've ever seen a skimmer at work it foams up, this is the proteins and salt mixing with air etc ... (you see this at the seaside too on windy days, sea foam is in effect proteins).