Like Matt said, that is a great shot. I really like the lighting, very moody.
As far as the modifier I mentioned? It can be something as simple as a piece of 8x11.5 paper on top of the tank. A diffuser is anything that has pass through capabilities. A good diffuser that I have had in the past is the Gary Fong one.
http://www.amazon.com/Gary-Fong-Coll...ng+lightsphere
I really wouldn't pay that ridiculous price tag though. I bought one used for about $30 and ended up selling it for the same price. While I did like it, and saw the benefits of it, I'd much rather shoot without another thing hanging off of my kit. It adds bulk to your overall kit that just plain got on my nerves. So I sold it. However, in your situation, you would be doing stationary flash positioning and it would work out well. You can alternatively, make your own diffuser .
Here's a quick thought, and keep in mind I have not done this AT ALL, but I do not see why it wouldn't work:
Get a small bowl, preferably some kind of rubbermaid tupperware. Cut out a slit that fits the flash head perfectly. Line the inside of said bowl with aluminum foil. Still use some kind of paper in between the top of the tank and the flash. This gets your flash off the tank a little more and spreads the light more. The paper helps the diffusion further. You can use wax paper as well instead of the 8x11.5.
Don't you have a modifier as already though? If so, make some kind of bracket and point it straigh down to your tank and see how that looks. With this last shot though, you aren't having clipping proglms at all. Looks great to me.