Hmm. That's a good question,  gut feel is that they are quoting the output of the emitters in air as I reckon they're just quoting based off the specs of the LEDs (which will have been measured in air).

I don't know for sure though.  Might be best to email the manufacturers directly? Put the reply up if you do! 👍

Thinking more about this,  although the wavelength will change, because the speed of light in the medium is slower, the frequency (therefore colour) will still be the same.

This now leads me on to thinking why aren't the light spectrum graphs for lights plotted against frequency? 

My original question would also have been  valid for people with glass or perspex lids affecting the wavelength of light....but I think you have answered it so thank you.

Ok, so I'm alright with physics but my biology is really weak. I've been doing some reading and this topic of lighting has led me to read about photosystem 2, which is the starting point for photosynthesis. It requires 1.8 electron volts of energy from light to oxidize water and start the process. This means it is not the wavelength specifically that counts in any medium, it is the frequency of light (therefore the photon energy).  1.8 ev is 680nm, from the equation e = hf.  This is red light in the visible spectrum. All the blue light that we pump into our tanks is excess energy. Please see      Light Reactions or many other sites on photosynthesis/photosystem 1 and 2/biology. I know blue light makes our corals look nice, but energy in wavelengths shorter 680nm is lost as heat as part of the energy transfer process to activate chlorophyll a. In summary, it feels to me like we should be aiming to provide a slightly fuller white spectrum to our tanks, rather than just aiming for the bluest of blue all the time. Red light is absorbed by water in the first few metres, but I doubt many people have tanks metres deep. So in our homes and in very shallow corals, light from the red end of the spectrum will play a part. There is a Japanese company that is producing true full spectrum lights that cover the whole visible spectrum, and I would be interested to hear if anyone knows much about the quality and results from using them. The company is kyocera. I'm sorry for the ramble, but if there are any biologists here who could point me towards some further reading on how photosynthesis works for a non biologist, that would be great.