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Add a Landscape, some vegetation. A Coral reef, maybe some east eggs as well.
I think that if you want it to be believable I think you should remove the betas from it. Betas are fresh water fish and they are dislocated in this scene.
You could work the background with some rocks and vegetation and corals as said @Magic-Mark.
Also you could fake some caustics, since the light there is strong enough for the viewers to expect to see them there.
If you decide to keep the betas and your objective is not being accurate you could give them some variation. They are all in the same position and in the same direction.
I like your scene.