I have these two gallon jugs (not two 1 gallon jugs, but more than one 2 gallon jugs)of water. The kind that you're supposed to use to pour water into cups at games. Well anyways, I just fill those up and pre-mix everything. Then I clean out the tanks, take the amount of water I need out (usually about 4 gallons or so in the 20, since it's NPT and the levels are really good and I have bottom feeders and the likes). Anyways, I put all of the water stuff I need in the 2 gallon jugs, and then I just flip the switch on the outflow of the jug to pour the water in. It's so much faster!
Why didn't I think of this before!? xD