Well, to be honest, you don't have many options since the portion of the elbow inside the tank does not have a hex on it.
I don't recall what the fitting is made of but the first option to try is to support the length of the elbow with a channel lock and try to turn the nut.
Another option would be to use an "easy out" on an extension-- insert it in the outlet on the inside of the tank, then attempt to wrench the nut off.
In a similar situation, I've use a center punch with a hammer to give the corner of the hex a sharp rap--"sometimes" breaking the nut loose.
Worse case scenario, you damage it--replace it.