I expected an old fixture with weak output. What I measured was a strong center and uneven distribution.
For roughly six years I assumed the original ADA Solar RGB was wearing out. Red plants toward the ends of the aquarium were not colouring the way I expected, and the fixture had been running daily through several rescapes. Replacing it seemed like the obvious next purchase.
I bought an Apogee MQ-510 expecting it to confirm that. Instead of one reading under the middle of the fixture, I took twenty underwater readings at the planted positions.
The center of the aquarium still produced strong readings. Several outer plant positions measured materially lower. On this aquarium, in this mounting configuration, what limited the layout was how the light was distributed rather than how much the fixture produced in the middle.
The same readings were lopsided front to back, which sent me to look at the fixture itself. It was hanging about an inch off-centre. It had looked fine for years.
I expected a weak light. I measured an uneven one.
That result is the reason this site exists in its current form. A tank dashboard tells you what is happening now. What I actually wanted was a record of decisions that could be checked later — including the ones where I was about to spend money fixing the wrong thing.
The full survey, with the method, every reading and an explicit account of what the numbers do not establish, is published separately.