First off, I disagree with your lfs!
Yeah, you "CAN" use tap water, but you can also use creek water as well. That doesn't mean you should and it certainly doesn't mean it's a good idea to, because it's not a good idea.
The reason why the ocean has so much waste but yet is still filled with beautiful life is because of its large natural ecosystem mother nature provided.
When you use tap water, you will likely encounter minerals and other ingredients which are good for humans but not as helpful for marine fish and inverts/corals. The water quality you start with will determine how successful your aquarium becomes.
And yeah it will help down the road when you slowly add RO water, but it's like giving someone a blood transfusion, yeah it works, but meanwhile, you're giving a lot of stress to the fish.
Just my opinion... :thinking: