here is prob the best combination for you. start off with getting the ro
unit. next look at investing into a phosphate reactor down the road. they help with excess nutrient if you have a heavy stocked tank and do daily feeding. if you do purchase a phosphate reactor it is best to accompany it with a protein skimmer to remove the excess nutrient the phosphate reactor secrets.(some research is needed to understand this if you use NPX bioplastics or other bio material). for such small tank i would stick with the RO
unit for now and see where things go. no need for a protein skimmer at this time especially for such a small tank. and yes weekly or bi weekly water changes is always a good idea no mater what type of filtraiton system you are running.