The skimmer size needed wil be dependent on the volume of the system, the ammount of livestock and if it is fish only or reef.

With some manufacturers it is a good idea to go for a larger skimmer as they rate the skimmer on the thoretical maximum size aquarium it could cope with. This will not take in to account stocking levels or real world situations.

Deltec give their skimmers a normal stocking and high stocking rating. These are accurate and take in to account what the skimmer can actually skim in a real world situation. Over rating a protein skimmer in this case may potentialy cause issues. They can start to skim ultra dry due to the low nutrient load, resuting in poor efficiency. In this case a smaller correctly rated skimmer would out perform a larger overated model.