Did you ask if he was getting commision from the sale of that water??
Seriously, It is very expensive but I know a few reefers that use it and like it......... but to claim that using it will reduce the frequency of needed water changes is preposterous! The number of water changes needed is determined on a case-by-case basis for each marine aquarium. Some tanks with a low-to-no bioload can go a looooong time without water changes; others with a high bioload would need frequent water changes. The frequency of needed water changes can only be determined by testing your water (not by the salt mix you use). If your water parameters are in check (ph, nitrAte, etc, etc,), then and only then can you postpone a water change. ME? I choose to perform regular changes to ensure that the animals are getting every mineral needed and to export nasties~~