We have a 564SS Greensmart 2 gas fireplace powered by a 20 lb grill tank which we had hooked up after the fireplace was installed. It works, but weakly. The flames are small and one of the burners doesn't even light. The fireplace installer says we need a bigger tank to get full performance. We don't rely on this unit for heat, just looks. What is the smallest tank we could get by with and still get a full burn?