I am lazy and measure TDS and ph. That is if I measure anything. I just do 2/3 water changes twice a day. I only have two tanks with fish so this is easy as both are drilled with valves to drain them. I also have fill lines plumbed to each tank. My water is very soft, 20 PPM TDS, and acidic. I run it through a calcite filter to protect my water heater. The water for the fish goes through the calcite filter. I used to have a geothermal heat pump that heated my hot water and had to have the calcite filter to protect the heat exchanger. If I ever get a large water aging tank the fish will get straight well water.