The best way to fill in an old septic tank is by crushing it and creating drainage holes. This method ensures the tank is properly decommissioned and securely filled. Firstly, the tank is crushed or broken apart using specialized equipment, such as an excavator. This process facilitates the tank's collapse and reduces its volume. Next, drainage holes are drilled at the bottom of the tank to allow for proper drainage and prevent any stagnant water accumulation. Finally, the tank is filled with a combination of sand, concrete, and gravel, ensuring stability and preventing future collapses. This approach effectively neutralizes the old septic tank while minimizing environmental risks.