do fish suntan?

I posted yesterday about my clownfish getting black patches, and one responder suggested it may be due to the lights. Do fish suntan?

In a related question, I have noticed recently that my yellow tail damsel is getting darker in color. He was bright blue when I bought him, then turned more purple, and now he is almost black (with a yellow tail). Why would this be? Might it be from the lights? What can I do about this?

 

under natural conditions fish avoid exposure to damaging UV rays by moving into deeper water or shade.
strong UV rays can cause sunburn effects at depths of .5m
a lack of kerotinised layers under the fishes skin and the presence of dividing cells making fish skin more readily damaged by UV rays, this is also compounded by the fact that fish epidermis does not normally contain protective melanin-containing cells.

basicly fish can get burnt but in the wild the sun is moving each day giving a vary or strengths or UV so that the fish can seek shelter or shade during the peaks.
in an aquarium the lights dont have a sun like schedual so fish obviously cant seek shelter all day so they get used to it (i.e. like us getting a tan) some fish are more prone to sunburn such as clowns and other damsels due to their bright colours absorbing more UV.

as for the aquarium to avoid your fish getting sunburnt dont use extreme amounts of lighting on shallower tanks i.e 400W MH on a 12'' deep tank an provide areas of varying light strengths so your fish can move around into different areas also provide plenty of caves etc so they can wind and run about.

i have a friend who has a pair of Ocellaris Clownfish and they were wild caught from a deeper area and when introduced to his reef setup with strong lights the top part of the clowns turned black. and as they got used to the light they moved into shaded areas and lost their "tan"

as for your damsel becoming darker, yellow tails tend to become darker as they mature. i am saying all of this with the assumption that he is not sick and that your water quality is high

hope that answers the question