From The Aquarium Wiki
What is Black Water?
Blackwater refers to water in rivers which is coloured dark brown or black and runs through forests. The colouration is caused by the tannins leached from the wood and leaves in the bottom of the rivers. Blackwater has slightly different properties to regular clear water, the pH for instance is often significantly lower, can go as low 3. Fish from these areas occasionally can be brought to live in regular tap water which is usually around pH 7 but would much prefer softer water. Some which live in the extremes will not tolerate harder water, their natural environment can be replicated using peat in the filters, Blackwater extract in liquid form, the addition of bogwood in the tank, some leaves such as Oak and Indian Almond Leaf work too.