I'm using K sulfate @10 grams per liter. It's the first thing I add to my stock solutions and the last thing to dissolve! After I make stock solutions I keep a high powered pump in the bottom of my tank that continues to agitate the mix for a few days before I use it. IME K sulfate always leaves undissolved salts behind- mostly dark impurities. My stock tanks are not blackened and I could improve light penetrating them.