Do not fill an aquarium with water just after the purchase. If an aquarium has a frame and glass junctures are covered with putty or sealant, then let such an aquarium settle for a couple of days, change the water once a day. During this time, all toxins from putty are supposed to dissolve in water. Then wash your aquarium thoroughly, fill it with water and leave for at least a day. And only then put fishes into it. The water level in the aquarium drops over time... this is...