What size UV watt for 90 gallon

I'm starting up a 90 gallon reef tank. I watched some BRS videos about there UV sterilizers. I wanted to get the 90 gallon one 25watt Emperor aquatics...but the guy helping me with the UV sterilizer set up talked me into getting a 18 watt one. He claims that a 25 watt would be over kill and that the UV should be based on the amount of overall water volume in the tank. Essentially he is saying that with LR and everything it will not be 90 gallons of water in the tank, therefore only needing the 18 watt. Anyone have anything to add to this. Should I get the 25 watt. I plan on running the sterilizer on the low gph end...not the higher flow as well if that helps.