Single Marineland is a load of #$*(, to be frank with you. I wouldn't use it to light up a goldfish tank.
The 36" fixture has more LEDs than the 24" so it makes a big difference. It's the only fixture that I truly enjoy from Marineland. Also have the 24" units on some 20Ls that are low light moss & crypt tanks and they're fine for that. Could even use two of them on a 20L without a problem.
But the 36" on this short tank is a problem if you don't want to run CO2 and dose EI. ADA (a forum member) ran into problems with his and had to raise it. I've had to do the same with mine. It's 16-18" (I'm guessing - I would have to look at my journal to know for sure) from the substrate at different areas of the tank. Crypts, E. parkeri, Marsilea minuta, Bacopa australis and Anubias are all doing swell with it raised high.