After looking at your image I think there are several things that may have conspired to make this shot less than the ideal one you see in your mind's eye.
First, the light areas on the side of the fish are pretty blown out, or waaaay too white. That could be the result of several things.
Start by checking the settings in your camera for exposure compensation and contrast. If the exposure compensation has been moved toward the bright end of the scale, adjust it back to the middle. Check the contrast setting as well and make sure it's in the middle of its range.
From the amount of noise in the image (the tiny confetti-colored dots most noticeable in the dark areas), it appears that the ISO setting for that shot was way too high for the actual conditions.
When shooting outdoors on a sunny day, your ISO control should be set to Auto or to the lowest numerical setting available on your camera. High ISO settings are intended for low light situations like indoors, at night, and the like.
Once you've confirmed that those settings are adjusted properly, getting a good picture in similar conditions can still be tricky. Here's why:
The light meter built in to your camera is set to adjust the exposure so that the overall light and dark balance in the image is about 50%. If you think of pure white as being 0% and pure black as 100%, an overall balance according to your light meter would be 50%, or a medium gray.
A scene or subject that contained huge areas of dark shadows and huge areas brightly lit by sunlight, would result in a photo containing nearly black areas for the shadows and blown out whites for the brights. Yet according to the camera's meter, the shot was perfectly exposed, even though the difference between darks and lights (contrast) would cause most people to consider the picture unacceptable.
No matter what kind of software you use, there's no way to add detail into blown out areas that's not there in the first place. Like the missing detail on the side of the fish in your image.
Fortunately, there are ways to compensate.
First, try to take shots that don't have large areas of shadow or highlights. If possible, shoot in the shade or on a cloudy or overcast day.
If that's not possible, then try positioning your subject so that it's either all (or mostly) in the sun or all (or mostly) in the shade. Smaller areas that are either too light or too dark will seem less objectionable that an image that's half too light and half too dark.
Finally, try fooling your camera's meter by pointing it before you take the picture at an area that's lit most like the subject on the photo you're about to take. If the fish is in bright sunlight, then point the camera so that most of the frame shows sunlit areas and then press the shutter release button halfway down to 'freeze' the exposure. Then turn the camera back to your subject and push the button the rest of the way to take the picture.
As a last suggestion, taking good photos is a lot like getting spaghetti to stick on a wall. Throwing one piece might not work but tossing an entire plate is almost sure to yield at least one noodle sticking to the wall.
Whenever you're taking pictures, take LOTS of pictures. Doing so will dramatically improve your odds of having at least one turn out well. You can always throw away the ones you don't like. But if you only take one and it doesn't turn out, you'll just be left with your memories.
K