Mackeral tank as lens in Maryport

The circular fish tank acts like a huge lens. If you look carefully at the writing on the emergency exit sign (the green one) you'll notice that it is backwards, which is called lateral inversion. That's because the light rays have swapped sides as they have been bent through the lens. Writing about this has got me wondering about the focal length. Next time I go I need to use the equation 1/u + 1/v = 1/f to calculate it.