What does a skimmer do?

[Aquaboi]

What is a skimmer for?

 

[Ingy]

A skimmer is used to remove proteins from the water. By injecting tiny air bubbles into a water column the proteins (which break down into ammonia etc) firm the 'skin' on air bubbles on the surface. They then bubble over into a collection cup.
By removing these proteins (poop and food waste) at the start of the cycle you reduce the eventual nitrates in the tank.

 

[Aquaboi]

Oh okay so i should get one for my 30g?

 

[Terrance]

For such a small system, you are better off doing more water changes in a given month. 5g per week is good.