We have a 4000 gallon septic tank and it does not smell and I do have some sense of smell
The reason it's OK is that air is forced in to it via a small 70W compressor 24/7, and this adds oxygen to the tank and so the bacteria are aerobic rather than anaerobic. The compressor lives in the garage and the air is transfered to the tank through 20mm plastic water pipe.
The other item we use is made by a company in Hampshire, called Flexford Brook Ltd and they make a product called Liquid Bio-Flo, found it works very well.