T5 Users, A Question about height.

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For all you dedicated T5 users. How high off the surface of water do you run your lights? If you could also include how deep your tank is, and how many bulbs the fixture is that would be appreciated.

I run a 4 bulb tek fixture with the bulbs about 6" off the water. Tank depth is 18". the reason I am asking is that I am finding that my birdsnest is bleaching out on me, and is quite low in the tank (12" under) iv'e tried it higher, and found the tips burning off of it.