Twenty underwater PPFD measurements mapped against the planted layout.
How is usable light actually distributed across a 90P under a single pendant fixture?
The tank is 90 cm long and lit by one pendant fixture that is considerably narrower. Plant placement had been decided by eye. Before spending anything on a different light, it was worth establishing what the current one actually delivers at each plant.
Light would fall off toward the ends, but the working assumption was that the fixture was simply underpowered for the tank.
Recorded before measuring, so it can be shown to have been wrong.
An Apogee MQ-510 underwater quantum sensor was held level at each of twenty repeatable positions corresponding to plant groups, mid-photoperiod, with the aquarium light at full output and room lighting minimized.
Distribution, rather than output, appears to be what limits this layout. Center readings sat inside the working range used for demanding red plants while the end positions fell well short of it.
A single measurement under the middle of a fixture does not describe what plants at the ends receive. The original premise, that the fixture was too weak, was not supported by the readings. Coverage appears to be the constraint with the current fixture. Mounting height was the first no-cost adjustment tested. Per-position measurement is now the basis for evaluating the layout.
Results describe this specific aquarium under the conditions recorded above. They are observations, not universal recommendations.