We are in the process of installing several giant rubber water tanks. Generally the center is filled with cement and the water is turned on. The guy selling these tire tanks has found a company making a big hard plastic 'plug' for these tanks. The selling point is, if you ever want or need to move the tank it's possible because you don't have all that cement in the middle. These plugs look like a pie pan turned upside down. We have put a 2" plastic pipe drain in the middle of the first plug and can't get it to seal. When the tank is filled the weight of the water pushes the plug downward and breaks our silicone seal causing the drain pipe to leak. These tanks hold 1800 gallons of water. Do any of you have any ideas on what we're doing wrong or what we can do to compensate for the weight of the water? Thanks.