those are not good skimmers. neither are coralife skimmers. you are wasting your money with those.
and to answer trackfasts question about what size of tank can you maintain without a skimmer, the answer is- any size tank. you can keep a 1000 gallon tank without a skimmer. you will have to remove excess nutrients some way though, so whether it's large water changes, mechanical filtration, chemical means....it's gonna be something.
a skimmer is another component that helps you to keep the tank clean.
in my experience, it's something you shouldn't skimp on. that said, for a tank under 30 gallons, i would just be religious with my water changes and skip the skimmer.