yes, they can drown, because they live by absorbing the oxygen in the water with their gills, without the oxygen, or in a low oxygen place, i beleive that they would drown, but i don't know for sure, i just suspect as much. Its like this, if people are in a room that we can't breath well in, cause theres no good fresh air, and we've pretty much breathed all the good air, all the oxygen and stuff, then we would suffocate, so in water, if the waters low on oxygen, the fish would drown. I dunno if it would be called drowning, but yeah.