How to repair the leaking tank in Solar Water Heater

by Anna
(Perth, Australia)

My Solar Hot Water Heater(made by Solar Kleen about 5 years old)is leaking from the small black cylinder that is on the top of the main tank. It has a hole the size of 10cents coin on one side at the top.It looks like it was always there but perhaps it should have a plug or something that came off and the hot water is leaking through this hole or is this some other problem that makes the water leak through this hole. Or perhaps the should be the hole.The water is not hot. How can I fix this?