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)...