Using peat as a filter material in aquaristics makes sense if you want to achieve water parameters that are as soft and acidic as possible for your stock. This can also be achieved with a CO₂ system, but not all aquariums are lushly planted, and not all ornamental fish tolerate high CO₂ concentrations equally well. Peat has further advantages as well, which we go into in more detail below.

What is peat?

Peat is a natural product that is harvested in bogs. Bogs are wetlands in which dead plant matter is only slightly decomposed in the absence of air, in other words practically preserved. Over thousands of years, thick peat deposits gradually form from the biomass. The properties of peats differ greatly depending on the bog type. Peats from fens and transition mires are generally richer in nutrients and minerals and more alkaline than those from raised bogs, where peat mosses of the genus Sphagnum play an important role. For aquariums, the acidic, nutrient-poor raised bog peat that is rich in humic substances is used and processed into peat products. Depending on age and degree of decomposition, a distinction is also made between the younger white peat and the older black peat.
Because bogs are valuable and endangered habitats, peat and peat products should be used as sparingly as possible.

What can peat do?

Peat as a filter medium offers many properties that benefit ornamental fish that love soft water in particular. Many species from South America, such as some tetras or discus fish, need such water parameters. Peat acidifies the water and reduces both the carbonate hardness and the general hardness of the water. This achieves very soft and acidic values and can even reach very acidic ranges that are difficult to achieve by other means. Peat acts as a cation exchanger and works on a principle similar to active soil in the aquarium. We have already explained in more detail here how soil substrate or a weakly acidic cation exchanger works.

Another important property of peat is that it enriches the aquarium water strongly with humic substances. At the same time, you have to accept a more or less strong brown tint of the water. It comes very close to the original habitats of several fish species and therefore contributes greatly to their well-being. Overall, humic substances have a very beneficial effect on the health of aquarium inhabitants (we have already taken a closer look at this topic in this article). The antibacterial effect in particular is an important property, which, for example, prevents fungus on eggs and reduces the germ load. The animals' entire immune system is strengthened and supported by the addition of humic substances.

The humic substances introduced also act as natural chelators. Important plant nutrients such as iron are protected by these chelate shells and are thereby available to the aquatic plants for a comparatively much longer time.

Humic acids as a buffer

Besides the lime-carbonic acid equilibrium, which describes a relationship between carbonic acid, carbon dioxide and calcium carbonate and serves in aquaristics as a buffer system for stabilizing the pH, other substances also play a role that buffer the pH and can consequently prevent a pH crash. These include above all phosphates, but also humic acids. The presence of humic acids in the water establishes another buffer system, which, however, buffers the pH much further down than the buffer based on the lime-carbonic acid equilibrium. Stable pH values down to about 4.5 are thus possible, and even with a carbonate hardness approaching zero, the risk of a pH crash is low, since the peat and its humic acids act as a buffer. Filtering through peat is therefore an ideal way to set suitable water parameters, especially for animals that prefer soft water values but do not tolerate a high CO₂ concentration well (such as discus fish).

Peat pellets as a ready-made filter material

Peat in a convenient pellet form is offered by various manufacturers and can be used directly as a filter material. Since the material expands in water, peat pellets are usually pre-soaked before use so that they can expand first and do not clog the filter later. Peat becomes exhausted over time. The pH-lowering effect and other effects wear off over time. Peat as a filter material therefore has to be renewed at regular intervals.