I think I pretty much mix mine the same way. RO/DI water that I store in 33 gal garbage cans. Aerate it overnight with powerhead and heater, add salt the next day, let it mix for a day, then adjust as needed. I usually have to add calcium, and I used to add buffer, but I realized if I aerated the water over night before adding salt, the alkalinity was usually up where I wanted it to be after adding the salt. Does that make sense? I think it has something to do with the excess CO2 like Joel said, or I just had a really crappy batch of salt... I just switched salt brands, so I'm hoping I won't have to add calcium either. I mix up enough to last me for like 2-3 weeks of water changes, but I have a little tank.