Drain the tank of water to below the level of the hole to make patching the hole easier.
Select a toggle bolt that is slightly larger than the hole in the water tank. Toggle bolts are machined screws with wings that spread open once the bolt is inserted. These wings act as an anchoring system. Toggle bolts are most often used to bear heavy loads when hanging things.
Select a drill bit this is slightly larger than the bolt.
Drill out the hole in the tank so it is uniformly round and smooth on the edges.
Slip a metal washer onto the bolt, sliding it up to the head of the screw.
Slip a larger rubber washer onto the bolt, fitting it against the metal washer.
Insert the toggle bolt into the hole in the tank so the washers are pressed firmly against the tank side on the outside of the tank wall.
Screw the bolt securely into place, but not so tight that you break the wings on the toggle. The objective is to firmly press the rubber washer against the edges of the hole, sealing the hole against water leakage.