Before you go out and start spending money, lets understand a few things:
When it comes to lighting, there are two things to consider; PENETRATION and SPREAD.
Spread is the area light covers at the BOTTOM of the tank. Think of a flashlight turned on and resting, light down on a table top.
As you pick up the light from the table, the area that light covers increases, the further you are from the table. This is called light SPREAD. And you can adjust the spread of your light by raising its mounting point above the water line.
A generic light from amazon doesn't have the necessary penetrating power to get to the bottom of a 36" tank, particularly if you have a mount point 8" above the water line.
PENETRATION is light intensity at the bottom of the tank. You need a PAR or LUX meter to measure that.