Welcome to AA!
If it were my tank, I would suggest starting the tank with a piece of raw shrimp (probably don't need the whole thing since your tank is only 20 gallons). You can leave the tank and it will just do its thing. If you throw some live rock in there with your base rock your tank should cycle pretty quickly.
I have used and recommended the raw shrimp method many times and it just doesn't get any easier IMO
. The arguments I see here against cycling with a shrimp are not based in reality. You're going to come home to a pretty nasty looking tank, but it will probably be cycled by the time you get home and you can just do a big water change before stocking. If you have not done so already, I would suggest purchasing a test kit so you'll have an idea of how quickly your tank is cycling.