Although I certainly cannot prove this......I believe Gill curl is caused by a prolonged exposer to a low PH. I dont think ammonia and nitrites have anything to do with this.....They effect the whiskers big time but not the gills. Everyone gets very defensive when their RTC has gill curl and always respond just as you have. Check your PH, I bet you'll be surprised. Being it takes a month or two for the curl to develop I havent seen many/any 2-4" RTC's with it, They simply grow too fast and by the time you notice it there usually 6-8". This is just my opinion and obsevation so dont take any offense, It happens very often. I personally have NEVER seen this reversed and have tried myself to do so. I have a real nice 14" guy that I rescued that has one side real bad,one side perfect. Ive never seen him breath heavy or have any signs of distress. He's just a little............ugly.
bob