I have personally tried the one at New world park (Better don't mention the name of the shop).
I thought it was fun having fishes nibbling my feet though I didn't find anything great after that.
Some of my friends have always have the thing about the hygiene, about sharing the same pool of water (like swimming pool), and it's proven that the fish spa is as dangerous as public pools.
Kinda scary after reading from here.
It's a true story I guess.
Lucky thing, I'm used to public pools, longkang waters or rivers. :D
But, if you were to ask me to PAY to do it again, I doubt it!