If you are going to have a reef tank you will not be able to use tap and have a stable tank. Your LFS
is just making a sale. There are many things in tap water that your magic potion can not take care of. Heavy metals, silicates that are causing your diatomes and all it`s doing to your ammonia is binding it up. It`s still there but you just cant read it any more. Maybe you are not doing a reef and then you`ll be OK but if you are do yourself a favor and get at least RO
water and RO
would be prefered.