The New England Aquarium has a touch tank where you are allowed to reach in and touch the fish - mostly rays - that are swimming by. You get to notice there that you can well see them when they swim but that they are near impossible to spot when they settled into the sand ever so slightly.
This one is quite easy to make out. But check out this flounder in a different display.
Would you have made this one out if there was no side lighting? I was having fun photographing the little guys. They seemed so curious!