I'm sure some fish see well in the dark and others don't. You're right that it's species related.
Regarding fish droppings, there is IMO
a correlation to a fishes health. White or clear feces might be indicative of intestinal parasites which can inhibit a fish from getting proper nutrition. Some fish regularly shed their intestinal linings and it appears their feces is white or clear.
I wouldn't panic, but it is something to watch for. Excellent observation on your part.