We consider compound lenses consisting of a thin glass lens with focal length $f$ and a thin water lens with one flat surface and one concave surface with radius $r$. According to the lens maker formula, the focal length of the concave lens is $-r/(n-1)$. Consequently, the focal length of the compound lenses is $f'$ satisfying the equation
$$\frac{1}{f'}=\frac{1}{f}-\frac{n-1}{r},$$
i.e.
$$f'=\frac{rf}{r+f-nf}.$$
Light rays parallel to the principal axis leaving the aquarium converge to the point at the distance $f'$ from the compound lenses. Assuming that the aquarium is sufficiently large in the direction of the principal axis, light rays parallel to the principal axis entering the aquarium are refracted by additional water to Point F at the distance $nf'$ from the compound lenses. (That is, $n$ times as far as the distance from Point A from the compound lenses, where light rays would have converged had there not been additional water.)
