I have a 150 gallon freshwater tank that wouldn't stay clear so I purchased a protein skimmer to remove the DOCs. It started pumping out a nasty yellow liquid on the first day. A while later I read protein skimmers were only for salt water and will not work for freshwater. I add one tablespoon of salt to every 5 gallons and this is what makes it work it also prevents fungus and ich. Unfortunately salt will kill live plants and invertebrates.Hopefully anyone with messy fish will benefit from having a protein skimmer.