Total internal reflection can also be known as the principle of the reversibility of the path of a ray.
The refraction of index of the solution in the tank will be measured with respect to either the air or glass of the fish tank. The index of refraction of the sugar solution is the relative index to the air or glass, depending on which is actually used in the experiment.
Here is an interesting website about total internal reflection. When you look at this java applet you can see how some of the light is always reflected off the glass. There is no point when all the light is transmitted through the water. One part of the light always has to be reflected, even through surfaces with very transparent substances (water and glass). The amount of reflected light is usually just a part of the incident light. The inclined the angle is the more the light will be reflected. There is a certain angle then that all the light will be totally reflected. This angle depends on what the two substances are.
Total reflection can explain why some objects can be shiny and also transparent sometimes become opaque and dull. An example is powerdered glass, this is a mixtuee of glass and air. The reflecting faces are now in all directions which throws the light all around making it diffusive.