If you put your CO2 tank in the refrigerator, it is unlikely that you have a leak. CO2 is a gas, and acts like a gas when stored. That means if it is kept warm, the gas expands and will give you the 700 pound reading. If you placed the tank in your refrigerator, where it is cold, then the CO2 will contract. So, it is not uncommon for a person to get a CO2 tank filled, place it in the fridge, and the pounds go down. Don’t worry, it is just the gas compressing. You will still get the same amount of CO2 as if you left it outside the fridge.
If you suspect a leak, simply fill a spray bottle with soap and water. Then, spray any connection between the CO2 tank and the keg. If you have a gas leak, the soap will start to bubble and you will know where to look. Most times you only need to tighten a fitting down to resolve the problem.