For fish, “soft” water means water with low TDS (Total Dissolved Solids). But it’s important to note that a test like the gH test is only testing for certain things in the water, namely calcium and magnesium, that are part of “general hardness”. These are the elements that make water “hard” by standard definitions, but they don’t include all possible dissolved solids. So it’s not really a full picture of your water’s TDS. Your water could test very “soft” on a gH test because it doesn’t have much calcium or magnesium, but be very high in TDS because it has lots of sodium, phosphates, potassium, fertilizer runoff, etc.
For our use drinking/bathing/etc when we consider water hard when it has a lot of the “general hardness” minerals that make it unpleasant like magnesium and calcium. To make water “softer” for drinking/home use, we use minerals that actually make the water have HIGHER TDS. If you are running a water softener in your house, what it actually does is swap out the magnesium and calcium for sodium and it does this at a rate of TWO ions of sodium per ion of magnesium or calcium. So this means a “water softener” is actually producing water with almost 2X the TDS of the water that goes in! This means this water is usually WORSE for your fish than the water from your tap.
Also, your water softener is taking out the gH and kH from your source water (and replacing it with sodium), so generally it is replacing "good" things for fish with "bad" things for fish. Calcium and Magnesium are good to have in your tank especially if it is planted.
Now if the hardness of your source water is super high, your only real bet to resolve this is RODI water. But most fish can handle the water you acclimate them to.
You could also pick up a TDS meter to see a real demonstration of the ion exchange process I'm describing:
http://www.amazon.com/HM-Digital-TD...UTF8&qid=1395261383&sr=8-1&keywords=tds+meter
Check source water softened, and through bypass. I would expect a lower TDS from bypass.
My strong recommendation: skip the water softener.