Dan, purple would be great. But a dark, dark purple. I think that Wet Canvas recommends it for shadows, not black. dd, a couple of things. Yeah, I agree that properly kept aquariums are nice and clean. But you are not dealing with a properly kept aquarium here. You want people to get an impression that the aquarium is much bigger on the inside (heh, how many times I've read this phrase :-)). So, increasing the murk a bit would help, in my opinion. Also, you don't have the difference in lighting between the aquarium and the room to help you there. Then, on the second thought, I don't recall ever seeing a home aquarium with such a steep ravine in it. If you could fill it a bit towards the foreground and then allow it to deepen towards the sub... BTW, I don't have PS Elements, but if it has Filter -> Render -> Lighting Effects, that's what you need. What you might do is to duplicate the layer, apply the lighting you want where you want it and then erase the rest of layer with a soft brush plus increase the transparency of what remains.

-- erlik