I will repost that there is much discussion around what is actually being cut. Perhaps a picture would inform the discussion.
Not speaking to what the OP is experiencing but to my own experiences. I've had issue with angle fish fin development in the past and trimmed the fins to allow for proper growth this short term pain dramatically increases their swimming ability and quality of living for the animals.
With regards to fin rot it was sighted several times, pristine water, clean instrument, stress coat etc. If he sticks by these recommendations he shouldn't have a problem at-least I never did.
I don't condone cutting fins for "cosmetic" reasons. That is silly the fins are the fishes legs and if you can help them experience a better quality of living by operating on them its worth the discomfort when you consider the bigger picture of the fishes life and viability with potential mobility issues.
Again allot of the speculation as to the legitimacy of the trimming in this thread is just that, speculation we don't know what we are dealing with there being no picture.
Maybe I'm not so opposed to operating on animals since I'm either assisted in or preformed small amounts of medical upkeep on many animals. Everything from trimming a dogs nails (something considered "cosmetic" that would apparently cause allot of pain by the dogs reaction) to lancing tumors on turtles or filling down their shells so they don't cut into their necks. Maybe I'm making bad connections here just saying I've found legitimate medical reasons to trim fins. Its not a bad practice inherently.