Most modern petricola (or types sold as this) are tank bred for a number of generations now so water type gets less specific, more a case of not moving from one extreme to another, but this bit goes for all fish and is easy to forget when generally all water looks the same. Soft acid to hard alkaline is the easiest move, especially for a fish which should prefer those conditions, but still not a given. A move back could do more harm than good after this amount of time (straight away, yes, but not now). However, I would probably either put him in a quarantine with a 50/50 mix of both water or add 10% of his old tank water when doing a 30% change. This second option I would only do if I didn't have a quarantine, as you bring a small risk to all the other tank mates.
This is just my advice from 40 yrs of getting it right and wrong and others may disagree.
If you can only get hold of one liquid test kit the big thing is to check ph (mainly) but not using strips as they are not accurate enough, and a swing of around 1 will require an hour slowly dripping water into a quarter filled bucket until it's full. Anything more than a swing of 1( especially from/to opposite sides of 7 will take a day (longer is better) so will need an extra tank. The strips are fine to give you a rough guide if anything is extreme and as such are useful if you have lots of tanks, but if you only have a couple or want to move fish from one to another it is worth doing liquid test kit test to check both water is similar.