Neglected Mer and inexperienced owner whumper 
Owners who think they don’t filter their tanks, so whumpee is just swimming in their own filth, it getting harder and harder to breath.
mer who’s owner thinks they just need a bowl barely big enough to turn in.
Owners who don’t put any hidden spots or enrichment into their tank.
The cramped cramped conditions both making them terrified of anything, or everything and depressed because they can’t move.
Owners who move them wrong, who just plop them in a new tank, without acclimating them to their new water. the shock being abrasive and painful it might even be hard to breathe if the salt lever is wrong
owners that don’t think salt water, and fresh water are that different. so they put salt water met into a freshwater tank or vice versa. The water suddenly becomes painful and making it hard to breathe.