Doing a water change doesn't always instantly solve all problems with water quality & the problem of not having a test kit...
How do you know the quality of the new water or the parameters you're trying to improve if you haven't got an exact knowledge of what you're working with.
I prefer not to use trial & error when it comes to animal welfare... But that is just me :-/ after all these little guys can live a very long time with the right care... or they can be a disposable pet that is lucky to last a few tortured weeks as a "pet"