I will answer because it is here..
the angle does 2 things, 1 spreadss the light out...
and spreads the light out.
spreading out will mean it covers more of the aquarium, but spreading out means it will be less powerful... so the lumens will be covering more space.
a 120 degree angle, means 60 degrees each side, if you think of it as a triangle.
( I cannot believe I am going to do math at this time on a Sunday)
a 90 degree angle, on an 18" aquarium, assuming the fixture is in the middle, will have about 9" that isn't lit by the LEDs directly , 120 degrees would give you about 5. but maybe 2/3 of the amount of light.
you can raise the light, or move it to one part of the tank and tilt it as that may help too.
I know I prefer to use 2 lights over a 40B, FWIW.