@jond6121 welcome to the tribe! The tire should not be losing that much air that fast, so there could be a couple things going on. First and most obviously you may have a hole in the tire. As you can't see or hear a leak, fill it back up to 20PSI and inspect it again; see if there's any green slime building up at any point on the tire, if so that's the slime trying to fill a hole and there's your leak. If no leak, then option 2 is your tire gauge is faulty; and if it's not that, then option 3 is you're losing air after you've topped it off. This often happens if the connector to the tire stem isn't pulled off quickly and cleanly after putting air in the tire. For me I like to ride at 20 PSI, so I fill the tire to 20.5 or 21 PSI, knowing that in that brief moment I disconnect the hose from the tire valve, some air is going to escape.