Just to provide some discussion, when I had a series of nano tanks under 30g without an ATO I constantly battled salinity fluctuations. Used a variety of skimmers that never quite got dialed in compared to a full size skimmer on a larger tank. Thus, most of the time they skimmed extremely wet further complicating the salinity issues which in turn complicated my water parameters. As I learned more I became diligent with trying to keep parameters stable which resulted in weekly water changes and perfectly matching conditions. Now that I reflect, I could have increased the tank stability by performing smaller water changes at least 2x weekly which is what I would suggest. Nutrient export never seemed as big of a deal as its difficult to have a large bio load due to the small tank size. Really my focus was the corals FWIW. I would save the money unless you can run a sump of some kind for a small fullsize skimmer. But in the end if you decide to upgrade none of the equipment can be carried over further reinforcing my suggestion of a focused husbandry/WC routine.
