Alright, well, you used a set of key words here.
You've got fresh water, and you want to adjust the chemistry of it.
Well, to tell you the truth, if we never had to worry about contaminants in the water, then
you'd never want to do any adjusting! 
My best solution, which is probably the cleanest option for anyone, is to purchase a Spa Frog:
http://www.rhtubs.com/store/spafrog.htm
Take a look at the instructions here:
http://spasupport.com/water
And reduce your chlorine or bromine levels proportionately to accomodate the Spa Frog mineral sanitization based system. There's just one caveat to those instructions at SpaSupport... test your water's alkalinity level before adding any kind of PH Balance. If you've got a high alkalinity level out of the tap, then you shouldn't need to add it... but watch it closely over the first few weeks, and then be sure you test the water weekly after that.
If you don't use a 'one quart fits all' solution like Ph Balance, then buy a large box of Arm & Hammer baking soda (LARGE), and take a look at the instructions on the side of it for maintaining your alkalinity level. You can use that (Sodium Bicarbonate), or purchase something similar to "Alkalinity Up", or "PH Up" from your local pool store, which is Sodium Carbonate, (not BiCarb), and a bit more effective in the long run.
What the PH Balance, Sodium BiCarb, and Sodium Carbonate do, is help maintain a buffer to take care of the residuals of oxidation that occur in your water that cause you water to turn to ph negative (acidic). Yeah, still got your head scratching right?
Ok, typical scenario, assuming NO chemicals in your water, and No Sanitizer:
1. Water's clean and nice.
2. Use your spa. Day 1.
3. Use your spa. Day 2.
4. Use your spa... Day 3.
5. Depending on your health, cleanliness, and environment, things start growing in that nice hot water. Perfect environment for it too right? (Dark, Wet, and Cozy!).
6. Go to use your spa... Day 4, Tub feels kinda slick... err... slimy. Things are growing.
7. Go to use your spa... Day 7, Water is green, starts smelling like a sewer. You don't want to even think about getting into this thing.
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Now we start over...
1. Use your spa. Day 1.
Add chlorine to kill everything.
2. Use your spa. Day 2.
Add chlorine to kill everything.
3. Use your spa. Day 3.
Add chlorine to kill everything.
4. Use your spa, Day 4.... now the water is irritating your skin. Not because of chlorine, but because the by-products of the stuff you killed with the chlorine is causing the PH to drop to acidity. (Below 7.2-7.4). There is no other alternative at this point but to drain your tub and start over..
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One last time, with proper 'Adjusting' done - What we do here is add the correct buffering agents (Alkalinity) to keep the water from drifting to acidic levels, and maintain sanitization simultaneously:
1. Spa Frog mineral sanitizer in filter cavity, Bromine or Chlorine Float in water.. water tests out with neutral ph (7.2-7.4) using test strips. Initial moderate shock (non-chlorine shock or chlorine shock depending on your choice of sanitizer) of water to ensure that whatever was growing in the pipes prior to your first fill (you did buy it used) is killed off hopefully.
Test strips indicate an alkalinity level of about 80 or less, (many places will read Zero), either add an appropriate amount of Arm & Hammer, Sodium Carbonate (PH Up from Pool store), <-We're talking tablespoons here... or a quart or less of PH Balance. Follow the instructions on the container!
In most cases, your stablized alkalinity level will be indicated within an hour or less. (You can use test strips immediately... then 15 mins later... whenever you want. Just be sure to check it sometime soon).
2. Let the spa sit and heat up. Test the water the next day to see that your Bromine/Chlorine levels match the requirements of the Spa Frog. Adjust the float vents accordingly to reach the required level. Check the bromine/chlorine - ph & alkalinity level again in the next 48 hours. You want to be within the 'good' ranges on all the scales of your test strips. With exception of Bromine or Chlorine PPM... Follow the instructions provided with the
Spa Frog. The information provided on this page will give you some good guidance with this product.
3. Use your spa day in and day out.
4. Bacteria and all the other stuff you don't want should be taken care of, and you should not have any problems.
Just be sure you monitor your water at least ONCE per week!, and use shock treatments judiciously!
There are no guarantees with this situation, so I only offer my personal and professional experience in this matter. Everyone's water coming out of the tap is different and will provide totally different results when dealing with PH and alkalinity levels. This is why it is important for you to test your water before you start, to check the alkalinity to see if you need to add any buffering agents out of the gate. (If you don't? Well, be sure you monitor this after your spa is in operation! Because you will later on that's for sure!).
Now, all that said, once you establish a routine, say you're using the spa three times a week every week... and all goes well, then all of a sudden you start having tub parties every weekend, you need to take appropriate measures that will protect the occupants. That is, apply shock treatments and much closer monitoring of the sanitizer, ph and alkalinity levels say every two - three days. And drain and refill more often!
Your tub with the correct circumstances will work and remain clean as long as you watch it. An investment of good test strips is cheap compared to the alternative.
Lastly, beware of walking into a corner store and walking out with a Visa slip and $125 worth of chemicals and stuff and not knowing what you're doing when you get home. I've seen this time and time again and it was THE number one reason that kept me in business repairing spas and hot tubs. Now if more corner stores could get on the same page with reality speak instead of 'sell a bunch of stuff to feed the bottom line'... the world of spas and hot tubs would be a much easier place - for techs and customers.
You get the picture... right?
Good luck and let us know how this works out...
