Hi Terence,
In our manuals we advise to hang the fixture at least 15cm from the water surface.
This has mainly to do with safety and reduction the maintenance (salt crystals on the glass plate), but also to ensure homogeneous lighting (in both intensity and spectrum).
For this reason we also never experimented with fixtures that are that close to the water surface and advise you increase this distance if possible.
Regarding the effect for your corals.
Having Corals close to your water surface is not an issue or a root cause for coral mortality.
Here an example in my own tank:
These corals are about 1-2 cm below the water surface:
But the distance between the light and the water surface is 18cm!
(practical example: you can see the light distribution pattern in the left backside of the image), tank width is 70cm).
The water surface (and especially a turbulent one) will act as a light ray distributor, scattering the light.
If the source is very close to this surface, the amount of scattering is drastically reduced.
This will lead to high peak intensities what go beyond the photo saturation level of most corals
In the middle point (at 25cm distance) the peak level is still around 1100 PAR.
But this does not seem to be the only cause of the coral mortality, you also suspect spectral uniformity.
The beam angle of the CoralCare fixture is about 60 degree (in practice even a bit more due to the light guide that has a Gaussian pattern.
So 7.5cm from the source, 15cm of LED’s overlap with each other (very rough calculation).
The complete balanced spectrum is achieved at total heights of about 10-20cm (depending on the angle between fixture and coral).
Together with the university of Wageningen we studied the effect of individual wavelengths (specifically 450nm (blue) and 665nm (red)
http://images.philips.com/is/conten...7_001-UPD-en_GB-Photophysiology-of-corals.pdf
To high irradiance of red (or in the Coralcare PC-amber) wavelength drastically reduces growth and eventually leads to Coral mortality.
So in theory a non-uniformal spectrum (due the insufficient distance between source and coral) could result in a “death Zone”
(what depends on the height between the lamp and Coral, but also the angle between them, perpendicular below a PC-amber LED would be the worst case position)