Hardness..How low is to low

If you were going to use the Tropical mix would only suggest using it at half dose - which will raise your Gh to about the 4 mark using the Carbonate hardness gen will raise you Kh and your Ph. Both will shoot your TDS probably round the 500us mark perhaps higher. There is a another way and will shoot you a PM which will probably get you to the mark that you want.