Coolant overflow tank leaking, how can I fix it?

Greetings,

My car was overheating then I realized that the fan was delaying and running for a small amount of time, then I managed to get a mechanic to install it to run together with the ignition. I have a problem, the coolant overflow tank it is leaking. The old one I was using started having cracks and leaking, then I bought a new one. It's about two weeks old now but has already got cracks and it's leaking as well. What could be causing the cracks and leaking of the coolant overflow tanks?

When I turn on the heater a bad smell comes out. What could also be the cause of the problem?
How can I fix it and how can I stop the leak?