For the broken back i think it randomly. But for this redtail catfish's tail (i took this pic from facebook) some random but most of the time they cut it so that it turn into heart shape cuz i already saw a guy who cut those tail. They also do it to datnoid, silver aro... I wont buy those with tail like that because it hurt the fish.
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Glad to hear you won't support that practice
They are such awesome fish! Just can't stand the thought of profit by mutilation.
O