Yep, everyone on line who repeats the same old BS you read everywhere about a protein skimmer not working in freshwater is lying to you by never even having tried it personally. I *have* tried it, and it works. It *is* busier than normal in this picture. The only difference between the freshwater and salt water is that fresh water is a more sterile, less filthy environment, so there isn't as much pollution to need cleaned out. In other words, the skimmer will only create effluent when there's something which needs to be removed. When nothing needs removed, it won't create effluent and basically acts only as a serious aerator, which is always useful to have. This is a Remora Protein Skimmmer, and I have it set for maximum, and leave it there all the time. If I have the opportunity to modify it at some point in the future, I'd arrange it to drop the collection chamber by about 1/2" towards the water surface so it would be more fully adjustable, but the self-limiting nature it has right now of stopping when there's nothing needing to be pulled out actually suits me quite well. Only the nasty stuff gets pulled out. What had occurred in this case was that I have an 2-part under gravel filter with an supply for each side. I just installed a new air pump on the half which had been down for while, and it went nuts like this for several days until all that stuff that built up was cleared out of the water. It also reacts like this on those rare occasions where a fish dies in a hidden location. I know I need to start looking right away if nothing has changed in the tank lately.