Most waters where Piranhas come from are very stained - like tea. If you watched Jeremy Wade catch a Black Piranha on River Monsters, you may have noticed the colour of the water. Consequently, Piranhas eyesight evolved to these conditions and they aren't used to being able to see as far as they can in our aquarium environments - I think this is part of the reason they spook so easily. I like to see my fish so I don't generally use the amount on the directions but start around 1/4 - it's easier to add more than to take it out. Water changes will remove it so it's not like it's permanent; just remember the amount you like and then top-up during your water change. I think it adds depth, character and mystique to the display as well.