MadSci Network: Engineering |
I suggest using white light. Red light is preferentially absorbed by water; that's why water looks blue. When white light goes through it, the red light is absorbed out leaving only the bluer wavelengths.
Anything that is warm will emit IR, but since fish are cold blooded they will emit as much as their surroundings, making them impossible to pick out. It's not that they don't emit IR, it's that they blend in with everything else!
In the end, you might want to call a photographer. Maybe you can try getting in conteact with a nature or underwater magazine; they may have photographers on staff that can help answer your question better!
Try the links in the MadSci Library for more information on Engineering.