Twenty underwater readings across a 90 cm tank under one ADA Solar RGB. Full method, raw data, the mounting-height test that backfired, and what a Solar RGB II would need to show to be worth it.
When someone quotes a PAR figure for an aquarium light, it is almost always one number taken under the middle of the fixture. That number describes one spot.
I measured twenty instead, on the 1st and 2nd of August 2026, with an Apogee MQ-510 held underwater at each planted position. The tank is a 90P, 90 cm wide, under one ADA Solar RGB I bought from Aqua Forest Aquarium around 2020.
I wanted to know whether my light was still good enough. Reds at both ends of the tank were not coloring the way I expected, the fixture is six years old, and I had more or less decided it had lost output and needed replacing. That was the assumption I set out to confirm.
I also wanted a baseline I could re-run, so that changing the height or the fixture means comparing numbers rather than memories.
Each item has its own page. Status and service dates there are read from the inventory that runs the tank, so they cannot drift from what is installed.
The fixture is a good deal narrower than the tank is wide. Keep that in mind for everything below; it turned out to be the whole story.
The ADA Shade is on the fixture and was on for every reading. ADA states it prevents light leakage and enhances downward illuminance by up to approximately 15%. Two things follow from that, and both matter for reading the numbers below. It is a manufacturer figure for illuminance rather than a PPFD measurement, and I have not tested it. And because the shade never came off, whatever it contributes is already inside every figure in this article — it is not a percentage to add on top.
The sensor goes in the water. Readings taken above the surface do not describe what reaches a plant.
One correction is worth being explicit about, because it is easy to mistake for a spectral one: a quantum sensor calibrated in air reads low underwater, and the MQ-510 compensates with a fixed 1.32 multiplier. That factor is applied by the meter, not by me, and it is the same on every reading here.
I have not checked it against a reference, so I cannot vouch for absolute accuracy beyond the factory calibration and the ± 5% Apogee states for it. Every reading came from the same meter held the same way, which is what makes them comparable to each other regardless.
Keeping the sensor level matters. Tilt it and the number changes without anything in the tank changing. I did this by hand, which is in the limitations below.
I measured at the plants rather than on a grid, which makes the numbers useful for placement but means this is not an even sample of the tank floor. The position numbers stay fixed between surveys so I can compare them later, one spot at a time.
Conditions held during the survey: aquarium light at full output with the ADA Shade fitted, room lighting minimized, water clear, readings taken mid-photoperiod.
Depth and fixture height were not recorded against the individual readings in this survey. The fixture was mounted at 12 inches above the water throughout, which is about 30 cm — the same distance ADA quotes its own illuminance figures at, and the mounting distance given for the current fixture. That height is a constant of this survey rather than a per-reading value, so it is stated here as a condition and not presented as stored measurement data. Depth per position is likewise absent. Both fields are captured by the logging tool now, and the next survey will carry them.
Every reading, exactly as logged. Nothing smoothed, nothing dropped. Pulled from the database at page load, so it is the record itself rather than a copy.
| Position | PPFD | Working range | Deviation |
|---|---|---|---|
| 01 Vallisneria nana | 176 | 30–50 | +126 |
| 02 Lobelia cardinalis 'Mini' | 90 | 50–90 | in range |
| 03 Alternanthera reineckii 'Mini' | 100 | 70–110 | in range |
| 04 Myriophyllum 'Guyana' | 70 | 40–70 | in range |
| 05 Myriophyllum mattogrossense 'Golden' | 70 | 60–100 | in range |
| 06 Rotala 'Orange Juice' | 135 | 70–110 | +25 |
| 07 Rotala 'Blood Red' | 155 | 80–120 | +35 |
| 08 Limnophila hippuridoides | 145 | 70–110 | +35 |
| 09 Pogostemon stellatus | 50 | 70–110 | -20 |
| 10 Hygrophila 'Kompakt' | 56 | 40–70 | in range |
| 11 Ludwigia arcuata | 80 | 60–100 | in range |
| 12 Hygrophila pinnatifida | 90 | 70–110 | in range |
| 13 Staurogyne repens | 105 | 25–50 | +55 |
| 14 Micranthemum 'Monte Carlo' | 122 | 50–90 | +32 |
| 15 Alternanthera 'Super Magenta' | 76 | 70–110 | in range |
| 16 Ludwigia glandulosa (Peruensis) | 102 | 100–150 | in range |
| 17 Rotala macrandra (FULL) | 95 | 100–150 | -5 |
| 18 Ludwigia 'Pantanal' | 95 | 90–130 | in range |
| 19 Rotala wallichii | 77 | 80–120 | -3 |
| 20 Rotala macrandra 'Mini' | 125 | 90–130 | in range |
The same numbers placed where they were taken. The table does not show you that the bright part is concentrated and sitting off to one side. This does.
The table above compares each position against a working range for whatever is planted there. Those ranges are practical reference figures from grower guidance, not published thresholds, so treat the comparison as a prompt rather than a verdict.
Two things came out of this that I did not expect.
The light is not weak. The middle of the tank reads inside the working range used here for demanding reds. The problem is that all of that intensity sits over a fraction of the width. What I have is a coverage problem rather than a peak-output problem, and I had that backwards for about six years.
Those two things are not independent, though, and it took me a while to see why. The lever for coverage is mounting height: lift a fixture and the footprint widens. But lifting it also costs intensity everywhere, which is what appears to have made the ends worse when I raised mine. It had no surplus output to spend. A fixture with meaningfully more output could pay for that height and come out ahead at the edges, though that is a prediction rather than something I have measured. Extra brightness is not the goal in itself; it is the currency you spend on spread.
The light was also hanging about an inch off-center front to back. I only found that because the numbers were lopsided and nothing else explained it. It looked fine. It had presumably looked fine for years.
Experiment 001 · PAR mapping · Experiment 002 · Mounting height
The obvious fix for a narrow beam is to lift the light. Raise it and the cone widens, so the ends should pick up. I did not want to buy anything before trying the free option, so I raised the fixture and re-measured the outer positions with the same meter, same time of day, nothing else touched.
The end readings went down. Not up.
Raising a fixture already narrower than the tank does widen the beam, but much of that spread appears to land outside the glass. You lose intensity everywhere and gain little where you wanted it. The light went back to where it started.
The usual advice is to raise a light if the edges are dim. On this 90 cm tank under a compact pendant, it appeared to make things worse. I have not found this tested anywhere with numbers, which is why it is here.
The raised-position readings were taken as a check at the time and were not kept as a separate labelled set alongside the standing survey. The fixture was returned to its original height on the strength of them, but I cannot publish a clean before-and-after table for this one, so treat it as a recorded observation that decided an action rather than as a documented result. Any repeat will be logged as its own survey under the current standard.
Measuring twenty spots instead of one changed my answer. The question I started with, whether the light was strong enough, was the wrong one. The fixture still produces strong center readings. The limiting issue in this layout is how unevenly that output is distributed.
That saved me from buying a light that would not have fixed anything, and sent me looking at fixture position instead, where I found a real problem.
I have not bought one. The reason I want to try it is specific, and it is worth separating who is claiming what.
The manufacturer's claim, stated as carefully as I can: retailer listings for the Solar RGB II quote up to 31,000 lux ± 10% at 30 cm, and give 30 cm above the water as the intended mounting distance. The figure of roughly 1.5 times brighter than the original circulates widely, and it is close to the ratio of the two published illuminance figures, but I have not been able to trace it to anything ADA publishes directly — ADA's own Solar RGB page lists LED count, wattage and colour temperature without an illuminance figure at all. Treat 1.5× as a retail claim of uncertain provenance rather than a manufacturer specification.
There is a second problem with that number, and it is the same problem this whole article is about: lux is a point measurement weighted for the human eye, taken 30 cm under the middle of the fixture. It is not PPFD, and it says nothing about what happens 45 cm to either side. A fixture could be half again as bright directly underneath and still leave the ends of a 90 cm tank exactly where they are now.
My measurements: the readings in this article, taken on this tank with this meter. They say the constraint here is coverage rather than peak output, and that raising this particular fixture cost more at the ends than the wider footprint gave back.
My hypothesis, which is neither of the above: if a replacement starts with roughly 1.5 times the output, that surplus may be enough to cover the loss from hanging it higher and still leave the ends better off than they are now. That is a question I can answer with the same twenty positions, and it is stated here so it can be shown to be wrong.
Brightness quoted at one point is not the same as usable light across 90 cm, which is the whole subject of this article.
So the first thing I would measure is not peak PAR. It is the center-to-edge ratio at a matched height, and then the same ratio again with the fixture raised to whatever height the extra output can afford. If the ends end up higher than the numbers in this article while the center stays reasonable, it worked. If they do not, that is worth publishing too.
Experiment 004 · Solar RGB II · How comparisons are run · ADA — Solar RGB · Solar RGB II retail listing and specifications
Experiment 004 · Solar RGB II · Experiment 005 · Canopy PAR · Project roadmap