When you say tanks, I assume you mean Corny Kegs. Here's a good way to trace down CO2 leaks. First, disconnect the CO2 connectors from the kegs. Turn the valve on the CO2 cylinder on and then completely off. You are now checking everything but the kegs. Any leaks will cause a fairly rapid drop in your gauges because without the kegs you're dealing with a very small volume of gas. If the pressure holds, you have eliminated everything except the kegs and their seals. Don't forget to check the O rings around the posts and remember that there can be cracks or pinholes in the keg itself.
If the pressure leaks down, turn the cylinder valve back on and use the Starsan solution to try and isolate the leak(s). If you are unable to find leak(s) using this method, Remove the cylinder, regulator, manifold if you use one and all of the CO2 lines and put them on the counter next your kitchen sink with the CO2 valve off. Fill the sink and submerge the manifold and line assemblies and turn the gas on. Be sure the CO2 cylinder is upright whenever the valve is on. You will see bubbles anywhere there are leaks. If you are using a manifold, turn the valves off and on while watching for bubbles. If you prefer you can use this method first. Sometimes getting all of the gas plumbing out of a kegerator or keezer is difficult, thus the earlier steps.
If you don't see bubbles the only things left are the regulator, tank to regulator fitting/seal, tank valve and, if the top of the tank is removable, the seal where the two parts meet. Check the stem of the valve on the tank as you open and close the valve. I have found leaks there as well as the stems on the manifold valves.
Take your time and be thorough.