I create a new layer and name it Piranha. You can add the water before or after. Of course, my piranha don't always need water, but that depends on your plot.
I select the airbrush tool (hotkey 'A'), click on "Fade" in airbrush options, and set it to eleven steps. I pick a nice orangey red color.
They're often schooling, so I make lots of strokes in the same direction.
Since so far they've been circling around in a tank, I select a slightly smaller brush and a slightly darker, greyer color, and stroke a few times in the opposite direction. Don't worry if they overlap.
Go to Filter-->Render-->Lighting Effects. Select Light Type-->Spotlight. Position the spotlight so it points dierctly underneath and is wide enough to cover all the piranha. You can mess around with the other settings (I like shiny and plastic), but make sure you've got lots of ambient light, and select a nice yellowy color next to the ambient light. Select Layer Channel-->Layer Transparency, check "White is High" and make it at least moderately mountainous.
Select an eye color, choose the pencil tool (hotkey 'Y'), and select a small brush. You don't need to give every prianha an eye. Just a few of those in the foreground. Select a tooth color (white may be too bright. Go a little yellow grey), and repeat with a point. Real piranha have jaws justting forward. It helps to zoom in (hotkey 'Ctrl-+').