A stupid question regarding protein skimmer

[mrhasan]

I run a rio protein skimmer and it is collecting good amount of "brownish" water in the cup. I was just wondering, the water that protein skimmers collect, is it just liquid waste or water from the tank with the junk? I do have to top-off with fresh water every 4/5 days due to evaporation but would also like to know whether skimmers removes some saltwater too.