They'll work in freshwater if the water has enough gunk in it and IF you can get sufficiently small bubbles (which is difficult). Foam fractionators were initially used in sewage treatment facilities.
Protein skimmers should start working around 1.006 if you have a heavy fish load. You may get skimmate that looks more like green tea than coffee sludge, however.
I made a co-current skimmer out of soda bottles as a test-of-concept. It was pretty inefficient. In a molly growout tank with the sg at 1.008, I was getting green tea skimmate. The stuff did stink, so it was pulling something out.
(For details, go to http://wetwebfotos.com/talk and search for "soda bottle skimmer". Plans are on my user page over there.)
Protein skimmers should start working around 1.006 if you have a heavy fish load. You may get skimmate that looks more like green tea than coffee sludge, however.
I made a co-current skimmer out of soda bottles as a test-of-concept. It was pretty inefficient. In a molly growout tank with the sg at 1.008, I was getting green tea skimmate. The stuff did stink, so it was pulling something out.
(For details, go to http://wetwebfotos.com/talk and search for "soda bottle skimmer". Plans are on my user page over there.)