when i first set up my tank about two months ago, i put the garden hose directly into the tank, added the directed amount of water conditioner, and then added my salt. All directly into the tank. waited a few days, and then started testing my water for adjustments to salinity. it took about a week to get it right because of waiting a couple of days every time i added some more salt to the tank (i think its easier to add salt than to dilute the water to get to the right salinity)..
I really didnt care about the water in the tank at this point because there was nothing living in it at the time. no live rock, no live sand, etc.....
Once my water was clear as can be (i used a carbon bag to get the pleasant yellow tap water color out), with everything dissolved and at the right numbers, i added some damsels (although i wouldnt add damsels again). I would suggest using a dead shrimp that is store bought.. you should be able to find some threads on here which discuss using this method.
After two weeks, i had my water tested for phosphates after seeing some diatom growth. and what do you know.. phosphate hell..... i added a phosphate bag into my sump and within a week or so, no more diatoms. At that point, i started adding alot of live rock, some cured, some uncured. but then had to take it all out when switching from crushed coral to live sand.
At this point in the game, i think you can do pretty much anything you like to the tank except for one major item!!!
If you plan on developing a reef tank, i would strongly suggest you get your tap water tested for copper (copper will kill your entire reef from what i hear). If it contains copper, you cant use your tap water at all unless you can figure out a way to filter it out such as using a RO/DI unit. From what i understand, copper will seep into your silicone seals on the tank itself, or into the rock/sand you are using rendering the whole tank and everything in it, Reef UNSafe.
If it doesnt contain copper, you can get most of the stuff out of it like chlorine, phosphates, either though carbon bags, phosphate bags, or lots of time.
But i agree with everyone else for the "after setup" solutions for topping off your tank with water. if you are going to use tap, then treat it and leave it sitting with a powerhead. even put a small amount of carbon and phosphate sponges in there to get those chemicals out if you are as anal as me.
Or, you can buy RO/DI water either from the start or for your top off from the LFS (about $0.50 from my local store). I hear some of the LFS even have trucks with large canisters that will deliver the water if you are going to get a significant amount. and they even have premixed salt water available too.
I would imagine your 125 will be similar to mine as far as needing about a gallon of water a day due to evaporation. So keep that in mind when you either buy or use your tap water as far as how much you are going to need to prepare before hand.
All the above is just my opinion as i am still learning alot of this information.