@saltyfilmfolks hit the nail on post
#5
Get a lux meter (or app) now – stop guessing and start measuring!
I would add that the question about height is a math problem. Triangle math.
I am not a math person, but I figured this out!
If you know the amount of area you need to cover (some or all of the surface of your tank, for example) and the lens angle on the fixture, you can compute the ideal mounting height so you have a good place to start.
It's easy if you have 90º lenses
In that case, mounting height = coverage radius.
So for a 48" x 24" (8 sq ft) tank, you'd figure about a 12" coverage radius. Mount your lights at 12".
For other lenses
Say you have 120º lenses over that same 48" x 24" tank. Break it down like this:
Code:
60º
|\
?? | \
| \
90º -----
12"
Make sense?
And use a calculator like this to fill in the blank:
https://ostermiller.org/calc/triangle.html
Mount that light at about 7" from the water to get a 24" diameter of coverage.
These numbers are very exact and lighting is very subjective, so remember these numbers are a starting point.