Here you can see that I pushed the colors back on the farthest corals,  eliminated the diver, added some spade fish and blue tangs and finally, added a foreground see turtle.   I also pushed up some of the foreground colors.   

This painting is for sale.  24" x 36"   $1,500.  

 As you can see, I've changed some of the coral outcroppings to add a few more varieties of coral.   Looking at this stage of the painting, I don't have the farthest  background coral reefs blue enough.  Because they are farther away and you are looking through more water, they should have more blue in them.   They are almost the same green color as the foreground coral croppings.

Art techniques

Here i have the background washed in with corals some reef structures.  Don't  be afraid to just start laying shapes in.  You can always change them as you go.   Also a pencil drawing can be useful to help layout the shapes before this step.