Uric acid and dissolved organic compounds
Waste products of fish metabolism that accumulate in water. Not measurable with standard tests, they are removed by water changes.
Detailed explanation
In addition to ammonia, fish produce a series of organic nitrogen compounds: uric acid, urea, amino acids, hormones, pheromones. These substances accumulate and are not processed by the classic nitrogen cycle.
Standard tests do not detect these compounds. The skimmer in marine aquariums removes part of it. Activated carbon adsorbs others. But no one removes them completely.
Water changes are the only sure way to dilute them. This is one of the reasons why water changes remain important even in aquariums with perfect parameters.
Fish pheromones and hormones can influence the behaviour of other fish. In overpopulated aquariums, concentration causes chronic stress.
The yellowing of water over time is a visual sign of accumulation of organic compounds.