The Groco Sweettank system is a holding aeration system that does work when installed, operated and maintained according to directions Groco Sweetank installation instructions . Except during winter or other extended layup, it MUST run 24/7/365 to keep the tank aerobic, even when you aren't aboard and that requires power to keep your batteries charged. So it's not a good choice if your boat is on a mooring or in a slip without shore power. Nor is holding tank aeration a cure for odor INSIDE the boat...'cuz unless a tank is leaking, odor in the tank has only one place to go: out the tank vent.
Aeration is rarely needed in tanks smaller than about 40 gallons...in most cases, some modifications to the vent to allow a better exchange of air along the with the right kind of tank product will do the job. I'll be glad to help you sort out what's needed and how to do it.
And before anyone chimes in with a suggestion that a fish tank aerator pump and a hose can do the job for a lot less money... Groco introduced the Sweettank system in 2003. If it were that simple, there'd be a dozen cheaper systems on the market by now, but it's still the only one...'cuz there's considerably more design and engineering to an aeration system that actually works than most people think there is.