FK
The difference is insignificant. If I remember Phillips numbering correctly, the 870 is just a little higher CRI (true whiteness) than the 865. Both are high and excellent for plants, IMHO.
KL
That's a great site! The 2 W/G figure bandied about on APD and in the Krib was a very rough rule of thumb, based only on standard small American tank dimensions. It was never advocated, to my knowledge, for any oddball tank measurements.
I was probably the first to point out that tank depth has nothing to do with the Wattage needed, since a tank is basically a light pipe that lets all light entering the water surface reach the bottom, minus an insignificant absorption factor for tanks under about 1000 G. Plant shading, algae on the glass and other factors are much bigger variables than absorption, but true light demand is most closely related to surface area than to any other parameter, just as your link shows.
Wright
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