Sphagnum or peat moss has a number of effects on aquarium water chemistry. It filters out minerals and releases tannins, softening water and lowering the pH. Many popular aquarium fish hail from water, called blackwater or cola water, with very similar chemistry. This includes many New World cichlids, tetras and some catfish. Adding sphagnum moss to you aquarium filter can produce the ideal conditions for these species.
The Right Stuff
You need to take care when selecting your sphagnum moss. Fortunately, the ideal moss is cheap anyway. The moss must be untreated -- don't buy from manufacturers who treat it with fungicides. Check the label to make sure the moss doesn't include additives. You can also purchase "jiffy-pellets," compacted pellets that can expand. Always remember, if the moss is expensive, you've bought the wrong type.
Using the Moss
To use the moss, put it into the foot of a pair of clean nylon stockings. Then, tie off the stocking and boil the peat until the water turns black. Pour off the black liquid that leaches out, then squeeze out the moss and rinse it. After this, you can add the still-tied stocking to an aquarium sump, hang-on filter or empty compartment in a canister filter. Here, it will slowly leach away tannins and absorb various minerals to create blackwater conditions.
Effects of Moss
Sphagnum moss has several important effects on water chemistry. First, it absorbs minerals and heavy metals through a process called chelation. These minerals buffer aquarium water, preventing the pH from dropping. So once they are removed, the water gets softer and the pH can drop more easily. The moss also releases tannic acid, which drops the pH. This closely resembles the waters of the Amazon and other rainforest rivers.
You can also use sphagnum moss as a substrate if you want to keep live aquarium plants. It will still alter water chemistry. Prepare it as you would for filtration. Then, spread it across the bottom of the aquarium. After this, add another substrate above it, like sand, gravel or soil. This helps prevent anaerobic pockets -- areas with low oxygen -- from forming. Anaerobic conditions promote the growth of bacteria that can foul aquarium water.