Pests and biological imbalances

A bacterial bloom is a rapid increase of suspended heterotrophic bacteria that turns water whitish, milky grey or slightly opalescent. It can be transient in a newly started system; in a mature aquarium it more often points to a sudden organic load or a disturbed biological filter. The colour itself is not the main danger: oxygen demand and concurrent ammonia or nitrite are.

Biofilter instability is a temporary or persistent loss of capacity to transform ammonia and nitrite. It has no single visual appearance and is recognised from data and timeline, not water colour. In a new tank it may reflect incomplete maturation; in an established tank it demands rapid explanation because nitrogen compounds can injure gills and tissue.

‘Detritus worms’ mainly include small oligochaetes and other free-living worms processing organic matter in substrate and filters. A hidden population is normal; thousands on glass or in the water indicate abundant food and detritus or loss of substrate oxygen. They are usually a husbandry indicator rather than an animal disease.

Hydra is a small freshwater cnidarian attached to glass, plants or décor. Its tentacles bear stinging cells used to capture microprey: a few polyps are often just part of the microfauna, while dense colonies can harm nauplii, fry and tiny shrimp. Risk therefore depends on host size, not visibility alone.

‘Infusoria’ is a practical rather than taxonomic term for small protists and other microorganisms multiplying in the water column. A moderate population belongs to the food web and may feed fry, but a visible bloom indicates abundant resources and can increase oxygen demand. Identification requires at least magnification or microscopy.

Copepods, ostracods and other small invertebrates are normal components of a mature aquarium. They become a ‘boom’ when density changes abruptly or they cover glass and water: this is a food-web signal rather than a disease. Meaning depends on the organisms, resources, predators and location.

Peritrich ciliates form stalked colonies that filter bacteria and can attach to surfaces, carapaces and already compromised tissue. Light colonisation on shrimp may be superficial; dense fouling indicates high bacterial resources and can interfere with gills, movement or moulting. Epistylis, Vorticella and Zoothamnium resemble one another but are not synonyms.

Freshwater planarians are free-living flatworms that glide over glass and décor. Some are predators or scavengers and, at high density, may threaten eggs, tiny invertebrates or freshly moulted animals; other similar worms are harmless. ‘White worm’ is therefore not a diagnosis: risk must be tied to shape, behaviour and host population.

A surface film is a thin matrix of microorganisms, proteins, lipids and dust concentrated at the air-water interface. An occasional trace is common, but a continuous layer means organic substances reach the surface faster than movement, skimming or degradation can remove them. Risk increases when the film limits gas exchange.

White biofilm on new wood is a soft, gelatinous or fluffy growth fuelled by soluble compounds released by the material. It includes bacterial communities and fungus-like organisms rather than one species. In most newly set-up tanks it is localised and self-limiting, but it must be distinguished from growth on living tissue or actual décor decay.