[HeyPK]

I am guessing that you took this picture without a flash, and that the lighting is the overhead tank fluorescents. You may have sacrificed some depth of field to get a faster shutter speed because not much depth of field is needed in this picture. the fish stays in pretty good focus even if the depth of field is reduced. The fish probably wasn't moving very fast. For more active fish, you have to use a strobe.

Way back, long before digital cameras were even thought of, I did some experimenting with a small strobe on top of the tank cover. I detrmined by trial and error what f stop I had to use for fish at different distances from the strobe. I had guppies, and guppies are always moving and turning. I swear, a guppy can not go more than two inches in a straight line, and it turns at least once a second. Most of my pictures were of a confused mess of guppies, all in the process of turning, and they looked quite distorted when frozen by the flash. About one in 20 of my pictures turned out better. Here is one:

 

[HeyPK]

I would say, stick with the overhead light because, if the fish isn't moving rapidly, you can get a good picture, as you did. You can experiment with the flash, but, even if you angle the camera so that you don't get a reflection off the glass in the picture, the fish will look sort of unnatural lit from the side. I took a quick look at the review article suggested by Jay Reeves, http://www.dpreview.com/reviews/olympusc700uz/page2.asp and your camera does have a flash synch connector for external flash. With a three foot synch cord, you can lay an external flash unit on the glass top of the tank and fire away.

Depth of field is a measure of how well things stay in focus at various distances further away or nearer the camera. With a wide-angle lens, almost everything is in focus from very close to the camera all the way out to infinity. With a telephoto lens, there is much less depth of field, and things too close or too far away, are blurred. Depth of field gets greater when you are stopped down to a small aperture, such as f-16 or f-22. Depth of field is less when the lens is wide open, like F-2 or so. Your Julidochromis was in nice focus because it was lined up in the plane of best focus. If it had been turned at right angles, pointed at you, its head and tail might have been a little out of focus, but its middle would have been in best focus. Notice in your picture how blurry the distant background is. Try taking a picture with the lens wide open and then another one with the lens stopped down to its maximum f stop. The first picture will allow a fast shutter speed, but will not have very much depth of field. The second picture will require a very long exposure, but will have much greater depth of field.

For a tank full of plants you want a lot of depth of field so that the plants in the front as well as the ones in the back are in good focus, and so one usually goes for F-22 and a long exposure time. The fish turn into blurs, but the plants look nice and sharp. Sometimes the fish almost entirely disappear in a long exposure. If you want the fish to be nice and sharp, also, then you need a big powerful flash unit so that you can get a lot of light in a very short time, freeze the motion of the fish, and still be able to have your lens stopped down to F-16 or F-22 for maximum depth of field.