1998 Mitsubishi Mirage Car has low idle

Hi, I was wondering about my '98 Mitsubishi mirage. It recently had a starting problem. The battery was dead so I tried jumping it and it jumped fine, but it would die out as soon as I put the car in reverse or drive (put load on it). Got the battery tested at an auto parts store and they said it was fine and that it was just half charged. So that's when I realized that the alternator was bad. I replaced the alternator and now the car starts up fine with no problem except for this new one:

Even if I'm sitting in a parking lot, the rpms get so low to the point where the car dies out. But as soon as the cooling fan kicks in, the rpms go back up (expected) and once it turns off the car's idle gets so low to where it wants to die out, and eventually does.

As I said, this just started happening as soon as I replaced the alternator. Do you think that maybe I put too much tension on the belt which in turn strains the motor? Or what can the culprit be?

Thank you for the help in advance

- ian