I assume for both. It just says it removes surface debris and improves gas exchange. I dont think it works like a protein skimmer. Probably more for freshwater.
Actualy I got one of those and it isn't too bad. I needed it for my 50gallon as there was a constant film on the surface of the water and it has taken that and broken it right up. Takes a bit of adjusting to get it right but after it was set up worked like a charm.
I have been using one for a few months in my freshwater tank (65 gal). Works pretty good at removing the surface film. Mine is connected to a 203 fluval. I also have a 305 fluval in the same tank.