Tracked it down. This is basically water in which peat has been boiled so that the tannins are released. It therefore tints the water, which acts as a shade against direct sun. It will also lower the pH because it releases acids. The effect of this will depend upon the volume of water, amount used, and the initial GH and KH of the water. The harder the water, the less pH will lower.
In aquaria, those who use peat add it to the filter. Same principle at work. It releases tannins and acids and stains the water and lowers pH to some extent depending upon initial GH and KH.
Softening the water is best achieved by diluting the source water. This is safer, easier, and more reliable. However, it is still not always easy. If you could provide the GH, KH and pH of your source water (presumably tap water), and what you want to lower the pH down to and for what fish, I might be able to help further.
In the interim, this article explains the hardness and pH relationship: Water Hardness and pH in the Freshwater Aquarium