Double NH3 trailers??

What size tires and setup do most guys use on their double 1500 or 2000 gallon NH3 trailers? I have P and H double 15 trailer with single tires on the back I think they are 16L something something size. Well I seem to need more tire as the back always wants to sink. Wondering if I should look at getting a trailer with tandem rear tires or if just going wider tires like a 21L would work. If you have pics of your setup please post. I will look at my tire size for sure tonight and try and post a pic also. thanks

Double NH3 trailers??