While protein skimmers are generally indispensable in the aquarium filtration department, they do remove beneficial things along with the bad. Along with dissolved organic matter, which later breaks down into nitrate and phosphate, skimmers can remove calcium and magnesium, and even cause the salinity of the water to decrease slowly over time. For calcium and most other beneficial substances, their removal is generally slow and over a long period of time, but it can happen. In terms of decreasing salinity, while this is also a slow process, if water changes aren’t performed regularly, then it could become a potentially big issue down the road. As the skimmer churns up the nasty brown goo, it’s also removing saltwater. Add in the fact that you’re constantly topping off evaporated tank water with fresh RO/DI water every couple of days, and your aquarium’s salinity can drop a few parts per thousand depending on the overall water volume of the tank. Again, this may not be much and the impact may be minimal, but it’s something to consider when getting that huge protein skimmer or deciding to be a little lazy with water changes.