Our group at school has been doing an investigatory project on how does fish live in different chemicals mixing with water (example: alcohol, evaporated milk and etc.). Now, I would like to ask on what is the significances of the fish living in different chemicals.. I need a lot answers thanks!:)

It would be cruel to torment fish by putting them in milk or alcohol.

I'm not sure what significance your teachers had in mind.

As for the biology, different chemicals each affect fish physiology differently. Small amounts of milk probably wouldn't hurt them, since it's just protein and sugar in water. A lot of milk would probably be bad. They like clean water because they run it through their gills to extract oxygen. They have to breathe the water they are in.

Ethyl alcohol is a toxin, so it kills nerve cells in the brain and elsewhere. It would sedate them, probably make them feel sick.

Here are some toxicology studies on Japanese fighting fish
Siamese fighting fish (Betta splendens) Chemical Toxicity Studies

Fish are sometimes used as indicators of water quality. Different fish have different sensitivities to pollutants and provide one way to monitor for or assess pollution problems. At a place I worked, our water output from one process to the local river had to be clean such that a certain minnow could survive in it. That may not be the current test, but it was a standard at the time.

More generally, a healthy fish population is an indicator of clean water.