How to clean hard water stains from gl aquarium?

In aquarium stores I have seen that the fish tanks are always clear without any hard water(salt) stains.How to effectively clean the stains?Can I use soft water(drinking water) to it or will the fishes die?I have two small black color sharks and one dalmation mollie.Even if I clean using vinegar stains reappear within 3 or 4 days.