Originally Posted by jgc
In addition to the refraction of the light as it enters the water (essentially creating a lens), there is now a water/glass rather than an air/glass interface which I suspect reflects more light (plus that light is at a different angle when it hits the glass causing greater reflection).
meh.. you can throw a lot of science on this, but home aquariums are not controlled science experiments. Any manufacturer will state their highest, most desirable numbers in a controlled setting, but it's really not 'real life' In the same way my car is suppose to get 40 mpg, but only get's 30. I guess the auto manufacturer forget that in real life people don't drive in wind tunnels, and they don't keep a constant speed of 50. And the biggest surprise is they actually have other people in the car too. On thing your forgetting is the tank the light goes on is not empty there will be hardscape and other plants in the way, there will be surface movement and water that isn't crystal clear like the air. Some light can also be reflected up and out or away from the plants. So give me PAR through air and an empty tank and I'll give you a Jeep that think's it's a Prius.