Mark out the lowest part of the picture, applying the Rectangular Marquee Tool and now we have to stretch it out vertically, using next option: Edit ? Free Transform (ctrl+t).



Move off the marking: Select ? Deselect (ctrl+d) and create now the layer's mask: Layer -Add Layer Mask ? Reveal All. Then apply the Gradient Tool of black color and make the visibility of the lowest part of the layer appearing smoothly:



Apply the next command: Layer ? Remove Layer Mask ? Apply and change the layer's Blending Mode on Screen.



Then make the correction on the color of the fishes' layer (Selective Color).