By “regress”, I mean essentially to de-evolve. The opposite of progress. Nature does not allow for backwards evolution. Everything moves towards natural adaptation. Never away from it.

A purely cave dwelling fish doesn’t need sight. So, over time- it will lose its sight in favor or other senses, such as smell or the ability to sense sound waves and motion. Also, eyes can be a disadvantage to them as they no longer need to devote energy to a useless sense.