For many species, the tint that comes from tannins in wood or leaves is a natural part of their habitat. Indeed, some people go to great lengths to produce the blackwater conditions that their pets would enjoy in the wild.
As there are no proteins involved in these compounds, they have no impact on water quality and are beneficial to a few biological processes, as well as revealing some of the colours and tones our fishes use to communicate and recognise one another in nature. When your water is as dark as tea, the luminous stripes of tetras or the high contrast patterns of shoaling catfish become all the more easily understood.
To reduce these compounds, wood can be soaked first for a long duration before being added to the aquarium, small pieces can be boiled to speed up the process or activated carbon can be added to the filtration to supplement a regime of water changes to dilute the staining.