Amber,
Twenty gallons is perfect! But if you are getting the black ones, spray paint them white or cream. Black absorbs heat - light colors reflect heat. Watch the soil temperature as the weather warms up. If the plants start doing poorly and not growing as quickly/well as you thought they would, insulate.
If you aren't using a timer for watering, try to be consistant with when/how often you water.
Daisy
