Bichir are not scaleless, in fact i have read that it is believed that they are the genetic bridge between fish and amphibians. There is a video on youtube you can watch about bichir evolution. Basically it goes on to show that scientists kept bichir in terrariums that were high humidity. What resulted was bichir that were able to traverse land with less slipage and more lift of the head. Indeed they were walking (kinda). What was discovered was the anatomy of the bichir changed, specifically the bone structure. They called this elasticity, a way for an organism to adapt to its environment.
Back to stress in bichirs to water quality. Yes they can breath air, but poor water quality would still cause problems with their tissue in the gills. Infections could develop. Bichir are known to leave bodies of water to find other bodies. I bet this is to find a body of water that isnt as harsh. Stress can stunt and even kill a fish, this is evident in the hobby as very few bichir reach the 2.5 or 3 foot mark they are known to reach in the wild in some species. Yes we are capable of keeping a fish in a healthy environment, but neglect can happen, and it leads to some bad outcomes in a moments time. There was a koi fish in japan that was kept by severap generations of a family in a pond, it lved for something like 225 years. So with non-neglect some fish can live extremely long lives they wouldnt normally see in the wild as well.