My apologies if I'm stating things you already know, I really don't mean to be lecturing you needlessly, but since I'm not sure what you know and it sounds like you might have missed a couple of these points:
Salinity is a measure of how much salt is dissolved in the water. When the water evaporates, the salt doesn't evaporate as well--it stays behind. Because of this, any container of saltwater will become more and more concentrated as the water dries up.
We know that every tank evaporates at least somewhat--yours, with fans, will evaporate rapidly. We know that the ONLY thing lost when evaporation occurs is water. Thus, if we want to put things back to the way they were when we started (same amount of water, same salinity,) what should we add to the tank? The exact same thing we had lost--water, and only water. Thus, adding pre-mixed saltwater to make up for evaporative loss only adds salt to the tank. Another way to think of it is to say that your tank currently has a pound of salt in it. With each saltwater makeup, you add another few ounces of salt. Before too long you suddenly find you have two pounds of salt dissolved in your tank instead of just one--now your salt concentration per gallon of water is much higher. Regardless of how much water is present, the amount of salt in the tank remains the same unless you physically remove it by removing saltwater in buckets.
As Greg said, to lower the tank's salinity, you need to remove salt by taking some saltwater out and replacing it with freshwater. To raise it, you would do pretty much what you've been doing--replace evaporation loss (which removes only freshwater from the tank) with saltwater. If you needed to raise salinity in a hurry, you would make an extra-strong batch of saltwater to add to the tank.
Follow Greg's advice to get your salinity back to normal--take out a gallon of saltwater, add a gallon of freshwater, let it mix (it will take time, probably 15 minutes or so, for the sump and tank to mix thoroughly) and check your salinity. If it's still too high, repeat, until it's at the right level. Once you've established the right salinity for the tank, from then on add ONLY freshwater to replace evaporative loss. You'll keep monitoring the salinity on a regular basis and make minor changes (by adding or removing saltwater) as needed.
Keep in mind that replacing evaporation is not a "water change." What we mean by water change is discarding roughly 10% of the tank's volume every week or so. In a 100-gallon tank, this means pulling 10 gallons out in buckets (to be dumped down the drain) and replacing them with 10 gallons of newly-made saltwater (since we just dumped saltwater, now we need to replace the same amount of salt we just took out.) We do this because over time, certain waste products from all the critters and bacteria in your tank build up--most of them don't evaporate either. Water changes assure that we remove those wastes that can't be removed by filtration devices.
1-2 gallons of evaporative loss per day sounds a little high to me, but then everyone's conditions vary. Why do you have the fans? Is it to keep temperature down? Not using them would be one way to slow down evaporation. Another way is to keep the lid on your tank and sump. Both of these will raise the temperature of the tank at least a little (evaporation is one of your tank's main means of cooling,) so if your tank is too warm, you'll just have to keep the fans and accept replacing 1-2 gallons a day, or look into purchasing a tank chiller (expensive.)
Hope this helps!
