Mushrooms might do okay in ultra low light conditions, and possibly a few other softies, but I'm not sure how they'd fare long term. Softies usually fluff out to gain more surface area in lower light, but one T12 (I'm assuming it's NO and not VHO?), is pretty low light for a 120. If it is an NO, why not add at least a second? You can buy some fairly cheap fixtures from Lowes (though, they will rust over time in a salty environment). I know Garf had good luck with Triton's and Blue Moons (also NO bulbs) over their tanks back in the day. Not sure if they still sell Tritons and blue moons, but it seems like GE had some decent NO bulbs back in the day, too. I'm sure they ran several, though, and don't know the depth of tanks they were running. For a little over $100, places like hellolights have retrofits that will give at least (2) T5's (not sure they sell VHO's anymore, but haven't checked).
I've got some sinularia/nephthea doing well in a 65 (~as tall as a 120) with only (2) T5's in a Tek fixture.