Eheim SUBSTRAT filter media, bioMECH or Synth filter wool for aquarium filters

The SUBSTRAT is made of highly porous sintered glass, a material riddled with microscopic cavities that offers around 450 m² of colonisable surface per litre. It is a purely biological media, designed for freshwater, to be placed in the part of the filter where the water arrives already free of coarse particles. One litre weighs around 830 g, which helps you check that your filter basket will support it.

bioMECH combines two functions. Its shape creates pockets that trap both coarse and fine particles, like a mechanical medium, while its fine pores house bacteria. It is neutral for pH, has no effect on hardness, and can be used in freshwater and in seawater. Place it at the head of the filter, or on its own if your filter has only one or two baskets.

The Synth wool is a fine synthetic wool, supplied in a 2-litre bag. Placed as the final stage, it catches the particles that the other media let through and makes the water clearer. Unlike SUBSTRAT and bioMECH, it does not serve as a lasting home for bacteria: it is replaced as soon as it becomes clogged, rather than washed, with no risk to the colony established in the other baskets.

The surface area of a filter medium cannot be seen: a litre of grow medium looks like a pile of small pale pieces, but each one is riddled with pores where bacteria settle. This is what sets it apart from simple gravel, of which only the outer surface is colonised. Flow rate matters too, because water passing slowly through the medium gives the bacteria more time to act, provided it stays well oxygenated. A clogged filter, on the other hand, lets the water bypass the media, which is why it pays to rinse the mechanical stage before it becomes blocked. To monitor the start-up, a nitrite test tells you when the colony is handling the whole load.