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)...
Save the fish.
If roaming more widely introduces them to more productive food opportunities (or, lower predation) than their closer ecology, then it would be beneficial for them. If it does not, then it wouldn't be. Neither context is determined in the basic finding that cocaine causes them to roam more widely.