Actually, once a year is too much, to have a septic system that's working properly pumped out. A septic tank is an ecological system that breaks down human waste into liquid, which then is returned to the earth to be absorbed into it and cleaned by it. As long as you're good to it, it will work for you. You don't need to put anything in it to make it work, it does that on it's own if you don't put things in it that will kill the bacteria. If you absolutely must put a helper product in the tank, break up a yeast cake and flush that down the toilet a couple times a year, that will be plenty.
Baltimore Sewer Service