it depends what type of hang on filter is it, do u know how many gallons it rated to do up to, also are you going to put liverock in your tank??? It depends on your budget but if you looking to spend as little as possible you best bet would be to buy a penguin biowheel filter for your tank you can pick one up pretty cheap... Also a protien skimmer skimes the protiend and surface scum from you tank along with other waste, im sure somone can explain what they do better then me, most people recomend them in the long run, it all depends what you want in your tank, how many fish, do you want a reeftank or a fish only??? Low end protien skimmers can be found on ebay which may be fine for only a 20 gallon tank...
