I'm new to kegging. I set my regulator at 20psi and have noticed it drops a bit every couple hours. There are no leaks that I know of and the tank is external if that helps. Any ideas why this is happening?
I hate to say it but you have a leak. Unless the fill on the tank is near empty as the beer carbonates neither gauge should be moving one set.
Get a spray bottle and fill it with StarSan or a dilute dish soap solution. StarSan is nice to use because it is also foamy but if you have to open the keg to reseat the lid soap won't be dripping into the beer.
Be liberal and spray every fitting. Around the lid and PRV. Under the disconnects around the posts. Spray the top of the disconnects. Spray each place where your gas line meets a connection. Spray where your regulator attaches to the tank. Spray the shut of on the tank itself (I've seen one of those leak before too!).
As you work past each part, move/rock them back and forth. Especially where the tubing connects to something. You may need to trim back tubing and reattach it. Be careful not to over-tighten clamps. Despite what your gut tells you that can also lead to leaks sometimes.