Water storage tanks can be made of various materials such as plastic or galvanised steel. With metal tanks, the water can slowly wear the tank thin and holes will develop. Catching these holes when they are small is your best bet for repairs. Once the leaks become large, repairs are more difficult and generally don't hold up as well. When this occurs, it is better to put your money into a new tank.
Purchase boiler repair plugs from a hardware store or home improvement store. These plugs are made of rubber and a screw mechanism. The rubber will create a seal with the tank. It works well for small holes. For tiny holes, use a toggle bolt instead.
Even out the hole by drilling it out with a slightly larger bit than the original hole. If using a toggle bolt, make sure to have the right size. With the wings folded, it should just be able to fit inside the hole. You want the wings on the inside. They will open up once pushed through and hold the bolt in place.
Place the rubber plug up against the hole and push it in place to plug the hole. For toggle bolts, push the bolt end with the wings through the hole, but leave the rubber washer on the outside of the tank.
Turn the screw mechanism to expand the rubber plug out. It will press up against the side of the water tank wall and seal it tight. Tighten the bolt down on the washer on the toggle bolt.
There is pressure inside a water tank and flimsy seals won't up under the force. Larger holes typically mean the area around the hole is fairly thin as well, which means that more holes will develop.