Could this not have been simply an instinct to find cleaner waters? I'm surprised they didn't add another control group which injected something unpleasant that could be naturally found in an area, but would be undesirable - ammonia, some sort of acid, or something along those lines.

The title ie a bit misleading:

The study want to prove that cocaine is yet another polluter thar alters the fish behaviour even in the small quantities that can be found in the wild in polluted areas. Not that something is special or different about cocaine pollution.

So the control group in this case are fishes with an implant with no drug at all.

https://www.cell.com/current-biology/fulltext/S0960-9822(26)...

At very low doses, for example chewing the leaves of coke instead of using the high purified version, it's somewhat like drinking a coffee [1].

I expect the fish to be more active. A coffee patch would be a nice 4th group as another control.

[1] Chewing the leaves of coke is common in many countries of South America, https://en.wikipedia.org/wiki/Acullico

Agree with your point overall, but ammonia in particular is a poor example.

Fish lack urea cycle, so they produce and excrete significant amounts of ammonia as part of normal metabolism.