nash: I believe the problem is white balancing. White balancing in laymen terms is basically adjusting to the kelvin temperature of the light source. In most cases we want to adjust such that all our pictures look like they would in sunlight. I believe sun kelvin temp is around 5300K or something.
I'm not sure if the D50 can manually set the kelvin temp, but in my D80 I can set the exact rating and then start shooting. It makes everything come out perfectly.
Our cameras also have AUTO white balancing, but they are really just guidelines and are never exactly accurate. In reef tanks they are greaty skewed because everything is lit in a certain kelvin by our lighting systems.
Another thing to do is research on how to use "grey cards". I'm sure there are tons of resources right off of a google search of "how to use grey cards".
Look for directions specifically for your D50. Learning how to search camera techniques is a great tool =)
Just a note: Photoshop can be thought of as a tool to correct mistakes made during taking a photograph, such as white balance, and such. Given that knowledge, our cameras have the ability to take the perfect shot, just takes some getting used to the settings to know how to make the perfect shot. But that doesn't mean you have to take a perfect shot, which is nearly impossible, and that's where photoshop can help you. But try not to rely on photoshop to correct everything, correct things on your camera first. Of course photoshop is also used to enhance and manipulate photos out of reality, so there are a lot of functions.
Sorry for being so abstract, shoot some questions here if you have more and I can give your more details. research first on white balance / grey card usage also, I think you'll find that to be amazingly helpful in our reef tank shooting and every other situation =)