I mix about 5 gal in a 18 gal tub and let it sit for 24 hours with a Maxijet 1200. The maxijet heats it up so I no longer use a heater.
I don't snap the top on airtight, but leave it on 2/3 to get some air circulating in it. I used to run an airstone, but I've been trying to make water changes as simple as possible.
Dana