Different what’re types have different buffers that controls the way water holds the ph and ppm. Starting with a neutral water like reverse osmosis gives you the best opportunity to feed exactly what the plants require. No worries of other junk or heavy chemicals. Also low amounts of nutrients will also make it hard to keep the ph stable. If the plant is using what it wants and leaves other stuff behind that will effect the abilities to buffer. That’s when your ppm knowledge comes in handy.