It's an optical illusion. The level you're using received a shock and are now unbalanced. The pix shown me that it has a lots of marks. Levels are very sensible tool that should be used with a lots of care. There's maybe just an half of a millimeter unbalance. That won't show to human eyes but makes a huge difference on the reading.
Edit : You (and your friend) didn't see that at the first place on a perfectly horizontal surface because even unbalanced, the mark are still in the perfect middle of the level.
That level needs to be replaced. Aint no way to fix that properly. You may gently tight the screws but... once it looses it never came back the way it's supposed to.
Sorry and good luck with your tank.
(God my english teacher never told me that one day I'll try to post about levels)
Edit : You (and your friend) didn't see that at the first place on a perfectly horizontal surface because even unbalanced, the mark are still in the perfect middle of the level.
That level needs to be replaced. Aint no way to fix that properly. You may gently tight the screws but... once it looses it never came back the way it's supposed to.
Sorry and good luck with your tank.
(God my english teacher never told me that one day I'll try to post about levels)