That’s a good reply. The only methods besides flashlight used for the photos of our book are a mirror (see frontispiece of “two” linke on the title page or page 148 (it’s one fish mirrored!) or nagyi on page 147 or rubrimontis on page 127. Martin Hallmann discovered that method to instigate aggressional behaviour and get the fish to open their fins to a maximum span. But in all cases there was no later playing with the computer for enhancing the colours.
However, we know from the internet quite a few photos where this had beed done; even some older photos of Horst Linke (see the old fourth edition of his Labyrinth book) have been “coloured”, but since years and in the new edition he did not do this again.