However, and this is just a however, it's going to depend on whether you are keeping your fish in the well water or the softened water. If you keep fish in 100% softened water, I'm afraid you'll be disappointed. You will need to get into adding back minerals, calcium and magnesium, that the softener just removed. On the other hand, straight well water might be too hard, that's why you're using a softener for household purposes. I would recommend, if you have one spigot somewhere (such as an outdoor hose) that is inline before the softener, mixing well and softened water. This is what my brother does. Experiment, and check hardness, until you come up with a good blend. Fortunately, the compostion of well waters are usually very stable, so once you find an ideal mixture, you shouldn't have to check it after that, unless you have some natural catastrophe that makes you suspicious of the well water.
HTH
judy