Biological Evolution outside of Earth
You take a population of fish and put them in a new environment (another planet even). Will they evolve?

Yes they would evolve as long as they don't get extinct. It sounds very likely that they would very quickly get extinct though.

Note, however that they were already constantly evolving while being on earth whether or not their environment was stable or changing through time. Evolution is not a thing that occurs only when the environment is changing. Stabilizing selection, genetic drift and mutations affect the allele frequencies too, not only directional selection.

General comments on the post

I don't quite understand the relationship between the first paragraph and the second paragraph.

The first paragraph suggests a few misunderstanding of evolutionary processes. I recommend that you have a look at a free online introductory course of evolutionary biology such as Understanding Evolution by Berckeley University for example. Typically, if some concepts I used in the second paragraph above were unknown to you, then you definitely want to have a look at Understanding Evolution