Light under water.

The nice thing about shooting aquariums is you can always go back and try the shot again. I generally shoot wild animals and you rarely get that second chance. In this case, the bubble tip anemone (Heteractic quadricolor) is a little out of focus and the maroon clownfish (Premnas biaculeatus), that is the main focal point in the anemone, is a little dark and blends in with the black background. A little more lighting on him would bring out his velvety maroons. You did a good job of contrasting the anemone against the background, now just focus on bringing out the fish. Also, try to get closer to your subject and wait for the clown to be facing the camera a little more.

Hello Samer

I could'nt agree more with Jeff. Take notice of his advice.

Greetings Richard