Here is a picture of the tank when it was first mixed. Here the refraction is very prominent and travels in a more orderly straight line. This is explained because although there are about three different substances in there each reside on their own level and they have not mixed around yet. Letting the tank sit in time, all of these substances will slowly start to mix around and begin to form many different layers with numerous indexes of refractions. All of these indexes would vary within a certain range. But a definite index throughout the tank is nonexistant.
If you have read most of my website, you should be familiar with a lot of the principles of light and refraction. If not, read all about it here.