Sulfur

Sulphur (abbreviation S) occurs in the aquarium mainly inorganically. As sulphate SO₄ it is dissolved in the water and can be absorbed by autotrophic organisms (sulphate assimilation).

Sulphur becomes dangerous when there is a lack of nitrate in malfunctioning denitrification filters, in compacted soil layers or filter substrate. Then sulphate is reduced to hydrogen sulphide (desulphurisation). This poisonous gas, which smells like rotten eggs, can combine with heavy metals, especially iron, to form metal sulphides which, for example, discolour soil layers black.

Sulphur is used elementally in the autotrophic denitrification filter (ADN) = sulphur-nitrate filter to remove nitrate.

Organically, sulphur occurs in some amino acids and thus also proteins and in vitamin B7 (biotin). In this form, animals and many bacteria absorb the required sulphur.