If you get good lights that are 1.5WPG T5 with good reflectors I think you can grow absolutely anything you want as long as you maintain good CO2 and dosing.
Andy
And on the flip side, you can also opt to have 1.5WPG of T5 lighting, possibly do little with regard to ferts and CO2,and
still have a successful planted tank. The growth will be slower, and you are more limited with your plant choices (less demanding species), unlike SuperColey's scenario, which does allow for more plant choices. I think you're on to something regarding the dosing, Andy. It's a simplification of a hobby that at times can seem ridiculously complicated with all the gadgets that are out there. I currently have 1.4WPG for a 20g and 1.8WPG on a 36g and the most I do for the 36g is 1 canister of DIY CO2 every week and once weekly, or whenever I remember, dosing of an all purpose fert. The 36g has power compacts, though, which may make a difference. I do nothing with my nanos.
I agree whole-heartedly that 2-2.5 WPG for a 31g is going to grow whatever you want, and you could probably have less.
Good luck to you, look forward to seeing this as a journal.
llj