Aqua Medic Multiballs Biological-chemical filter material for fresh and saltwater aquarium is available from Swell UK.
Multiballs can easily be separated so that chemical filtration products can be inserted. This would double the amount of filtration. Depending on the types of inhabitants in the aquarium, Carbolit, Antiphos and Hydocarbonate can be added, removing pollutants and toxins easily.
The Multiballs provide a large surface area of biological filtration as bacterial colonies can grow. So if chemical filter media were added, your tank would benefit from both biological and chemical filtration and super clean, healthy water.