Hi There…… A 20 gallon pot will be fine but you will need to water from the top for a while, to get the roots down to the bottom….. Also ensure you use a good substrate or blend of substrates to ensure good capillary lift….. We have used 33 gallon SmartPots in the past with fantastic results. The water will pass through the fabric no problem, so don’t worry there. I would also use an AQUAvlave cover over your AQUAvalve and not glue it down.