Gill curl takes a long time to develop in poor water or so we believe or know so far, I think.
I'd guess your case is likely genetic. The RTC we get are farmed and their genetics is overall sub par. Moreover, their raising conditions are probably poor too as they are raised as cheaply as possible, probably in SE Asia, perhaps in SA too, in the name of maximum profit, just like most things enterprising humankind does.
Then we come along and gobble it up, let it be food or ornamental fish.