Thanks for the good advice. My tank's pH is 7.4 when the lights are CO2 are off but goes down to 6.4-6.6 when the lights & CO2 are on. I read on some article that Discus don't like pH swings,how do I avoid the big pH swings ?

One of my tanks has a KH below 20mg/L, what do I add to effectively buffer the water? How can I increase KH? Do I add Seachem's Alkaline buffer?