invest in a rain barrel (easy to make) if you have the room to do so.
I make the recommendation only because,
I'm guessing if you are using 20 gallons of water this isn't an apt grow.
Actually with that much water you probably are going to need 2 of them.
I just put mine out a few weeks ago and off a pretty small roof (6x8 )
it's been keeping about 15 plus gallons in it with me drawing off it.