which one i need filter or skimmer?

[christyf5]

Saltwater tanks don't use mechanical filters like freshwater aquariums do. All of the biological filtration is done via bacteria living in the live rock. The bacteria break down ammonia, nitrite and nitrate via the nitrogen cycle.

In a smaller tank such as yours, it is easy for nutrient levels to get out of control. If you are diligent and perform frequent water changes and are careful with feeding you should be able to go without a skimmer. However if you are seeing excess algae growth you may want to add a skimmer to help in nutrient export. There are several different models of hang-on skimmers which would suit your tank. Its been a long time since I have run a hang on skimmer for a smaller tank and mine was a piece of crap so hopefully someone else can recommend something for you. I don't think you can go wrong with the deltec or tunze brands though.