To start, I am going to do some drastic manipulations to unblock the black areas and get some color into the image. First we will apply Filter>Stylize>Solarize. Then we will use Filter>Fade, and select Luminosity blending mode at 100% opacity. Luminosity blending mode returns some of the original color of the fish. Basically what we have done so far is to eliminate the bright highlights and to greatly reduce the luminosity of the lightest parts of the image. This is all in preparation for the next step, which is Image>Adjust>Invert, which takes the inverse of the image. Then we will use Filter>Fade, again in Luminosity blending mode with 100% opacity. Inverting the image makes all the very light portions black, and I didn't want large dark portions in the image. That is why I chose to mute those light portions of the image first, using the Solarize filter.

Now we have a very pale image, so we will use Image>Adjust>Levels to fix this. In levels, drag the left-most slider over to where the histogram starts, and then experiment with moving the middle slider around until you get the tone you want. You can see the results of all our work so far in Figure 2.

My next concern is to reduce the difference in color between the bottom of the picture and the blue background of the rest of the image. We will do this by using Image>Adjust>Replace Color. Replace Color is a very powerful tool that allows you to use the eyedropper to precisely choose a color in the image and change it to another color. There is a fuzziness slider for Replace Color that lets you adjust how wide a color range will be affected by the change. You can preview the results directly in the image, so you know exactly what you are doing. Before using Replace Color, we will first use the Lasso tool to select just the lower portion of the image. This is because we do not want to change the colors in the rest of the image. We will also feather the selection (Select>Feather), so that any change we make fades gradually into the rest of the image. I used a feather of ten pixels, but you can use whatever number works. Use the Replace Color eyedropper to choose the greenish color of the image foreground, and use the color sliders to make as good a match as possible with the blue background. You may have to use Replace Color several times to do this, each time selecting a somewhat different color range, until you are happy with the result.

Even after this color matching, a line was still evident, so I decided to add more fish to the picture to at least partially cover up the foreground area. I decided to make two copies of the fish, and to put those copies into the foreground. To do this, I started by making two duplicate layers (Layer>Duplicate Layer). One of this layers I flipped vertically (Edit>Transform>Flip Vertical), and the other I flipped horizontally (Edit>Transform>Flip Horizontal). Now, in the layer that was flipped vertically, I used the Lasso tool to select the area above the fish, I feathered the selection at ten pixels, and then I used Edit>Cut to remove those pixels from the layer. Then, I used the Move tool to drag this layer downward, and used Edit>Tranform>Rotate to tilt this fish at an angle and position it so mostly its head and one fin was still in the botton of the picture. Figure 3 shows the results of this layer over the background layer, with the visibility of the third layer turned off.

Now I selected the layer with the fish flipped horizontally. First I cut away the darker foreground pixels as we did in the previous layer using a feathered selection at 10 pixels. Then I shrunk this layer using Edit>Transform>Scale to make the fish about one third its original dimensions, and I used the Move tool to drag this shrunken fist over the lower left of the image, overlapping its tailfin with the fin of the larger fish in the background layer. I used Edit>Transform>Rotate to tilt the smaller fish somewhat, and I used Edit>Transform>Skew to get the width of his tailfin to match the width of the larger fish's fin.

I cut away the sharp edges of the top two layers with a deep feather to blend them into the background layer. Then I flattened the image.

In all this work using layers, be sure you always know what layer you are working on. If you accidentally make a change to the wrong layer, just undo, select the correct layer, and start again. It helps to turn off the visibility of the layers you are not working on. You do this by clicking and unclicking the layer's "eyeball" symbol in the layers palette.

Now there is still some cleaning up to do where the overlap of the layers is evident. I used the Rubber Stamp or Clone tool with a reduced opacity brush to copy pixels and texture from one portion of the image to another, until the image appeared seamless. As your cloning gets more detailed, switch to smaller brushes. You may want to use pen input and a Wacom tablet for the most detailed work. Also, you should use the Magnifying Glass tool to enlarge the image several times and do all your cloning on the enlarged image, for best results. One place to pay particular attention is the blending of the two fins. I used high magnification and a 50% opacity brush to join these two fins as if they were one. You can see the results of merging the three layers in Figure 4.

Now we are ready to work on the color. I used Image>Adjust>Channel Mixer, but you can use any of the color tools to experiment with the color. My final result is in Figure 5.

You can see that in the final image, it no longer appears as if you can see the bottom of the pool, and all that distracting bottom detail and the glare on the surface ripples of the water have been transformed into an interesting background texture somewhat like Japanese paper. Also, we have created an interesting composition of several fish, instead of a picture of a single large fish all by itself filling the image. We produced interesting and vivid colors, bringing out the brilliance of the fish. The shimmer of its scales is evident in the purple reflections, as the scales appear to pick up some reflected color from the water and some lighter magenta direct reflection from the sun. As we originally envisioned, the final result has the feeling of a painting or woodblock print.