I just got a second hand 450 litre tank.
It's been empty for 2 mths and when I got it I decided to test it just in case.
It was full for 48 hrs, no leaks.
But, I noticed "something" on the bottom seal.
It looks like a reflection or a bubble. But I'm not sure.
The tank seal is all one, so it looks like trapped air. But I wanted to get others opinion who know more.