Velocity... this is not freshwater fishkeeping with salt added!! It is NOT the same. If it were that easy to treat tap water and have it work in a SW tank, don't you think that's what we'd all be doing???
Tap water can contain lots and lots of stuff. Some you can test for, some you can't (unless you have a supersize test kit, and I'm assuming you don't). You cannot treat out some things that could be in your tapwater.
If you have copper pipes, you are introducing copper into your system. You may be introducing phosphates. Silicates. Nitrates. Who knows what other crud might be in your tapwater.
Oh, and a 72 gallon tank is not going to require 72 gallons of water. Assuming you are going to have a DSB and 70# LR, it's probably half that.
If you aren't willing to pay for the 35-ish gallons of water, be prepared to pay in other ways... fish life, algae headaches, etc. Personally I'd rather buy the water than watch my expensive fish and inverts kick the bucket.
Katie