Here is a step-by-step on how I make blackwater extract for my Bettas. Gouramis, Tetras, some Loaches, some Cichlids (GBR's come to mind), and many other fish come from areas of the world where leaves fall in the water and tint it. The water can become colored, anywhere from a weak tea color to a very strong coffee color. This extract can change your pH if you have soft and relatively unbuffered water. Many people believe this stuff is therapeutic to fish native to blackwater habitats. Some manufactured versions of blackwater extract can be seen selling for $10 for a small bottle. This stuff is free and fun to make. Plus your fish will love it!
Just add it to your aquarium until the desired color is attained and enjoy your more natural looking habitat.
Below are some pictures and in the description you will find information on the process.