Really nice fish there! I've thought about getting threadfins many times, but sadly they're not suited to any if my tanks
Are they in regular tap water?
Threadfins are one one the best rainbows to have as they suit a wide range of parameters, in particular higher pH and harder water but they will be fine in lower pH as low as 5.0 as well as soft water.
So yes, regular tap water with dechlorintor as normal.
I have had thread fins before, lovely little fish to have, lots of character and behaviour to see, especially in a single species tank with decent numbers in shoal.
Some more info on Threadfins Rainbowfish :
http/www.seriouslyfish.com/species/iriatherina-werneri/