A semicircular glass ball is centered on the outside of the tank. A sort of oil can be placed between the glass ball and the tank that way no air will be in between to refract the light. At all angles, as long as the laser beam passes through the center of the ball it will be perpendicular and the light will not refract through that interface. Then as it passes through to the fish tank it will be refracted.

This method is based on the principle to measure the index of refraction using a critical angle. There are many other ideas on manipulating the fish tank to achieve a critical angle. This is one of the reasons why this tank poses so many interesting problems. The critical angle cannot be easily achieved. The light beam has to be pre-setup by something else other than the fish tank.

The only problem with this idea is that there is a limited range of measuring the angle that the laser beam enters the glass ball. So this will affect how accurate the index refraction can be measured.