Thanks for all the input guys. I have since replaced the tank ($20 part) with a new one.
I will be keeping an eye on the tank in the future.
I do a lot of stop-and-go riding commuting to work daily.
The bike will almost daily get hot enough to cycle on the fans, so heat may be an added factor for me.
Also the direction I have been parking the bike at work tends to expose the top of the tank to
direct sunlight. I would have to assume that the tank is not rated for UV exposure.