For smaller than 20 gallons, you may want to consider using 5-gallon jugs. Instead of a power head to circulate the freshly-mixed saltwater, you could use an air pump/airstone to aerate for a day. Pretty cheap. Some units have dual output lines, so you could have 2 jugs aerating simultaneously.
I plan to do the same thing for a while for our 72 (10-gallon bi-weekly changes) until we upgrade to a 150 at the end of April. At that point, I'll be doing 20 gallons bi-weekly, so we'll use the BRUTE and a Maxi-Jet 1200 powerhead.