There are a few differences in your pic and the example. The example is using a direct light or flash from above. The fish is in shallow water but off the bottom. Can tell by the shadow being larger than the fish. The water level can be seen in the upper corner. Water will give a color cast, as well as a blue container. The more water that is on top the more it will affect the picture.
You don' thave to have fancy flashes for what you want to do. You could use 2-3 simple clip on lights with matching bulbs. Most light bulbs now a days comes with its temperature ratings on the package. 5,000k is basically daylight and would be a good choice to buy / use. But since your using digital you can adjust white balance to match the bulbs if you don't get 5,000k. Obviously a flash or two will work very good. But if your on a budget you don't have to have them.
I am guessing on your picture your white balance is off some. And it maybe be a bit under exposed. Both are probably caused by the blue container being used. Not so sure how much experience you have with your camera settings. There is a couple ways you can make changes. One is to shoot Raw (NEF on your D50) and adjust white balance later with software. Or you can chimp (look at picture on camera display) and take a sample pic look at its colors, and adjust white balance up or down accordingly with the camera WB settings. If you use the same set up all the time (same lights, same distances, same container, etc), you can probably write down the settings that work. And each time use those setting from then on. If anything just minor adjustments.
I missed you were using 5,000K shop lights. Might check your camera and try taking some pics with the White Balance set to Daylight. Then take a couple sample pics and if they look dark adjust your exposure for a longer shutter (can use exposure compensation), or jump over to manual mode and keep Aperature the same, but slow down the Shutter speed from the previous pic. Might want to switch over to center weighted metering. The all blue background is probably confusing the metering a bit.
Oh, Welcome to the forum.