Guide category
Common Problems Guides
Choose a topic based on the signs you observe in a freshwater, saltwater, or reef aquarium. These guides help you compare possible causes, decide what to check first, and plan safer next steps for livestock health, water quality, algae, pests, and maintenance.
Common Problems reference library
Common problems
Aiptasia Anemones
For an isolated suspect animal, confirm identity and prevent transfers. Isolate an intact removable item only when this will not damage living or hazardous tissue; choose any targeted product by its exact compatible label. 'Aiptasia' is a hobby name for pest anemones, not a photo-based species identification.
Common problems
Aquarium Algae Problems
'Aquarium algae' is a practical label for several photosynthetic organisms and look-alikes, including green films, filamentous algae, diatoms and green water. Cyanobacteria are bacteria. Dinoflagellates are a diverse protist group that includes photosynthetic and nonphotosynthetic forms; their aquarium blooms require a separate identification pathway.
Common problems
Asterina Starfish
“Asterina starfish” is a hobby name applied to several small hitchhiking sea stars, many now placed in the genus Aquilonastra. Species-level identification from appearance alone is difficult.
Common problems
Black Beard Algae
Black beard algae is an aquarium common name for dark, attached freshwater red algae. Hobby identifications often use Audouinella or related names, but appearance alone rarely establishes a genus or species.
Common problems
Bristleworms in Reef Aquariums
Bristleworm is a broad hobby term for marine segmented worms with bristles. Their diets and bristle structures differ. Some Amphinomidae, including Hermodice carunculata, consume living cnidarians; do not classify every worm as harmless or harmful from color alone.
Common problems
Brown Algae and Diatoms
Diatoms are microscopic photosynthetic organisms with silica-containing cell walls. In aquariums, many species form a removable brown biofilm commonly called brown algae, although they are distinct from true brown seaweeds.
Common problems
Cloudy Aquarium Water
Cloudy water is a loss of water clarity caused by suspended living organisms, fine solids, droplets, bubbles, or precipitated minerals. It is a symptom category rather than one diagnosis.
Common problems
Coral-Damaging Nudibranchs
Coral-damaging nudibranchs are shell-less marine gastropod mollusks whose diets may be highly host-specific. The aquarium label covers multiple unrelated species rather than one universal pest.
Common problems
Cyanobacteria in Aquariums
Cyanobacteria are photosynthetic bacteria. In aquariums, some form cohesive surface mats commonly called blue-green algae or red slime algae, although they are not true algae.
Common problems
Detritus Worms in Aquariums
“Detritus worms” is a hobby umbrella term for small free-living aquatic worms, commonly oligochaete annelids, that consume decomposing organic matter, microbes, and biofilm. It is not one species or a disease diagnosis.
Common problems
Dinoflagellates in Aquariums
Dinoflagellates are diverse single-celled eukaryotes, not one aquarium species. Reef nuisance blooms commonly receive genus labels such as Ostreopsis, Amphidinium, Prorocentrum, or Coolia, but reliable identification generally requires microscopy.
Common problems
Fin Rot in Aquarium Fish
Fin rot is a descriptive syndrome in which fin tissue progressively degrades. Opportunistic bacteria may invade damaged or stressed tissue, but similar fin loss can result from trauma, aggression, parasites, toxins, or systemic disease.
Common problems
Fish Gasping at the Surface
Fish gasping or piping at the surface is a sign of respiratory distress, not a diagnosis. Immediately provide safe aeration and confirm pumps, filters, and surface movement are working; test the water and review recent changes without delaying support.
Common problems
Freshwater Ich (White Spot Disease)
Freshwater Ich is caused by the ciliated protozoan Ichthyophthirius multifiliis. The feeding stage lives in the fish's skin or gills. Later stages leave the fish, reproduce on surfaces and release new infective cells.
Common problems
Green Aquarium Water
Green water is a high concentration of microscopic photosynthetic organisms suspended throughout the water column. Unlike film or hair algae, it does not primarily grow as an attached coating.
Common problems
Green Hair Algae
Green hair algae is a common name for many filamentous green algae rather than one species. They grow as attached chains or branching filaments and reproduce or spread through microscopic stages and fragments.
Common problems
Hydra in Freshwater Aquariums
Hydra are small freshwater cnidarian polyps related to jellyfish. A tubular body attaches to a surface by a basal disc, and tentacles armed with stinging cells capture tiny aquatic animals and move them to the mouth.
Common problems
Marine Ich (Marine White Spot Disease)
Marine ich is caused by Cryptocaryon irritans, an obligate ciliate parasite. Feeding trophonts are protected within fish epithelium; mature parasites leave, encyst as tomonts on surfaces, divide, and release infective free-swimming theronts.
Common problems
New Tank Syndrome
New Tank Syndrome is toxic ammonia and/or nitrite accumulation because nitrifying microorganisms are not yet abundant enough to process the aquarium’s waste load. It describes a system problem, not a contagious disease.
Common problems
Pest Snails in Aquariums
Pest snail is a name for unwanted aquarium snails, not one species. Common freshwater hitchhikers include bladder or pond snails, ramshorn snails, and Malaysian trumpet snails; each differs in shell, egg-laying or live-bearing biology, and behavior.
Common problems
Freshwater Planaria and Flatworms
Planarians are free-living flatworms that glide on a mucus layer and can regenerate after injury. Aquarium keepers also call many small nonplanarian flatworms “planaria,” including harmless rhabdocoels, so the common name alone is not a diagnosis.
Common problems
Red Planaria and Reef Flatworms
“Red planaria” is a hobby name often applied to rust-colored acoels such as Convolutriloba species, which are not true planarians. “Reef flatworms” also includes unrelated marine flatworms, including coral-feeding polyclads such as the Acropora-eating flatworm Prosthiostomum acroporae (formerly Amakusaplana acroporae).
Common problems
Swim Bladder and Buoyancy Disorders
A buoyancy disorder is failure to maintain normal depth or orientation. The gas bladder helps many bony fish regulate buoyancy, but weakness, gastrointestinal gas, body shape, organ disease, pressure injury, and water quality can mimic primary gas-bladder disease.
Common problems
Vermetid Snails
Vermetids are marine gastropod snails in the family Vermetidae. Unlike mobile snails, the adult cements an irregular shell tube to a hard surface and feeds by casting mucus threads or nets into the water to capture suspended particles.
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