I want to get the EcoTech Marine VorTech MP10 ES Propeller Aquarium Pump not sure if I really need the MP10 W Es or just the MP10 Es. Is there a major difference between the two? I have a 36 gallon tank, in the long run I can't see needing two Vortech's and from what I understand the difference is just the ability to link up other pumps. Is that correct?