Water quality vs fish color

Been fishing lately in a swampy area in C. Fla. Caught 'gill, shellcracker, catfish and bowfin- all were almost dark black except for belly area. Wondering if anybody knows what causes this? :dono

The tannic acid in the water makes it appear black and the fish take on the coloring of the water. Lived and fished the glades around Lauderdale and caught a lot of them dark fish. Caught bass that would be black as coal.

Thanks- It was bugging me, had to look at tail to see if was channel or bullhead cat. Gills a little easier,