I have soft water like you, and only add kH occasionally. I am very lean with my fertilization, though, so that may help. I have never had any issues with a tank crash, except one time when just starting, and it had not been due to pH issues (I put in too much dechlorinator when I switched brands and Bad Things happened). I usually try to choose things that work with my water. Admittedly, my current tank is a 20 gallon, and has been for years, so the larger format may buffer the tank better. I live in a city that publishes DEP water reports yearly, if you live in a city, you may want to check to see if yours does, too. They can give you some in-depth info that you may miss out on if you just have the API and Seachem tests.