So 40" x 16" x 19.6"
55gal
This is the Fluval size of err "best fit"
Fluval looks to have about 2.5cm dead space on each end.
Therefore the diode panel is approx. 31cm +/-.
So say light sources are about 4.5cm from the edge.
3cm from the light face to the top of tank is the sweet spot for left/right coverage.
10cm "up" off the water line to cover front/to back at the water surface.
Lets use my "rule of thumb" here for a minute..
@45.7cm from the light face (which we set at 10cm, so water depth is 35.7cm (14") PAR est. is 46-ish
In 2019 a Fluval 3.0 was measured using a Seneye.. and found this:
So 42 par @ 18" seems to be a fair ballpark. Better to error low than high.
So now you have an idea about 1 way to calculate setting up your light.
Oh and the use of this:
LED Lighting Beam Angle Calculator
As to "better" for err which reason?
Fluval gets a knock by some for having a more warm tone i.e yellowish.
Which can be adjusted but at the cost of par.
So next "step" is what 'shade" do you prefer?
Counter example to the Fluval yellow is the Beamswork DA fspec (non-adj, non-dimmable (out of the box)) budget friendly light
which runs quite cool. Like 10000k. Nobody would say its too yellow unless they had tons of tannins in the water.
If one wanted all around high par color adjust-ability they start to get more expensive than the Fluval 3 @ $200US
G3 Aqua air 900 is $299.99
But at 2x the wattage.
Direct from Micmol. Cheaper at Alibaba btw.
Has a funny channel layout but functional.
They state the 36" has 150-200 par at 14-ish inches
LEDs Power: 216 * 0.5W which is 108W
According to err "me" should be around 108PAR at 18" light face to 18" down.
Keep bracket height in mind.
"Better" is often defined by what you "need"..
Disclaimer: No lumens were hurt in making this post.