I personally don't recommend skimmers on anything less than 35 gallons. weekly 10% water changes is good enough for anything that you can keep in your tank. ten percent isn't that bad... and in fact, a 30% water change once a month wouldn't be bad either. you just have to pay attention to your tank. protein skimmers remove more dissolved organic compounds from the water column, but what matters the most is getting rid of the dead stuff within the rocks and on the surfaces and stuff that builds up in the filters, which can pretty much only be done through water changes.
if I didn't change your mind already, octopus makes decent hang on back skimmers for your size tank. If your on a budget, Eshopps makes a decent in-tank and hob versions of tiny skimmers suited for your system.
