A septic tank can breed thousand of roaches. A cracked septic tank can be the home base for roaches that infiltrate nearby buildings. Roaches can even crawl up into your home through your pipes, bringing along what filth fills the septic tank. However, even a small amount of insecticide can kill off microbes in the septic tank needed to break down the solids—necessary to keep the tank functional. Aside from being sensitive to the microbial life, you need to deny entry in the future, which may mean finding pipe or tank cracks where the critters got in.
Try rotenone, temephos, diatomaceous earth, or boric acid in your septic tank. The latter two work as desiccants, cutting the sides of the roach. Therefore, place them in the tank where they can stay dry for at least a few days, if that is at all possible. The first three won't kill the microbes that your septic tank needs. The first two can be flushed in.
Flush tadpoles down the toilet. They may take to the roach-filled environment and proliferate off the population. However, they might come back up through your plumbing occasionally.
Use a roach bomb, if you can get the fogger inside the tank. According to the manufacturer of the Envirocycle Septic System, it won't affect the needed microbes in a septic tank.