Diffusing the overhead flash normally helps with hotspots on fish who's color creates highlights. Simply placing a piece of white copy paper on the glass top and the speedflash pointing down through it can be enough diffusion.
Getting the correct white balance is another reason shooting in RAW format can be helpful. You can make the adjustiments in the post processing when the white balance may not be properly calibrated on the camera.
There are other issues with this shot that need to be addressed too IMO.
It appears to me that you cropped this image considerably from a larger composition. The tell tale signs are the high noise levels and pixelation.
After seeing the original (thanks for emailing it to me Mike), my suspicions are correct.
In the copy I made shown below, I left more background in the image. I adjusted the levels, tweaked it in curves, did a slight shadows/highlights, cleaned up the artifacts and garbage off the original and did a layer process to eliminate some of the noise. Also adjusted the sharpness with the unsharp mask tool.
I would normally not recommend doing this much post processing. Retake the shot until you get better results so less time can be spent in the computer lab. :smashfreakB:
