H
Fish health decision framework
Diagnostics & testing
Understand fish-health tests, sample quality and what results can—and cannot—prove.
20 practical guidesOriginal AquaNexus writingReviewed 2026-08-13
Decision-first guidance
Choose the situation that matches what you are dealing with.
These pages connect back to symptom and disease profiles where a narrower differential is needed.
Diagnostics & testingPreparing for an aquatic-veterinary appointmentA useful appointment starts before the fish leaves home. Bring a concise timeline, current water results, treatments already used, clear media and details of every fish affected so the veterinarian can spend time on diagnosis rather than reconstruction.Diagnostics & testingBuilding a useful fish-health case historyA case history turns scattered observations into diagnostic evidence. This guide shows how to record onset, progression, stocking, water, feeding, maintenance, new arrivals and prior treatments in a form that reveals patterns.Diagnostics & testingTaking diagnostic photos and video of sick fishPhotos and video are best for documenting change, breathing, swimming and lesion progression rather than naming a disease. The guide covers consistent lighting, scale reference, both sides of the fish and short behavioural clips.Diagnostics & testingCollecting an aquarium water sample for diagnostic workWater sampled after a large correction may no longer represent the conditions that harmed the fish. This guide explains why timing, clean containers, tank identification and recording temperature and treatment history matter.Diagnostics & testingWhy diagnostic sampling should happen before medicationMedication can suppress organisms, alter lesions, change water chemistry or make culture and microscopy harder to interpret. When the fish is stable enough, collecting appropriate samples first can preserve diagnostic value.Diagnostics & testingChoosing a live, moribund or freshly dead fish for diagnostic testingSample quality changes quickly after death. This guide explains why a veterinarian or laboratory may prefer a live affected fish, a moribund specimen or a very fresh carcass depending on the tests planned.Diagnostics & testingSkin scrape and mucus wet mount: what the test can showA skin or mucus wet mount can reveal some external parasites and changes in surface material, but a negative slide does not exclude every cause. Collection technique and prompt examination are critical.Diagnostics & testingGill biopsy and gill wet mount: what the test can showGill tissue can reveal parasites and gross cellular or structural clues, but the procedure can stress a compromised fish. This guide explains why it is usually a veterinary or trained diagnostic procedure rather than a casual home test.Diagnostics & testingFin and lesion wet mounts: what they can and cannot identifyFresh material from a fin margin or lesion surface can sometimes show motile organisms or fungal-like elements. It cannot by itself prove that every organism present is the primary cause of disease.Diagnostics & testingFaecal examination in fish: what it can and cannot showFaecal examination may reveal parasite stages, unusual material or digestive clues, but intermittent shedding and contamination can make interpretation difficult. Results must be tied back to appetite, body condition and species.Diagnostics & testingCytology and impression smears from fish lesionsCytology examines cells and surface material from lesions. It can support a differential by showing inflammation, cell populations or abundant organisms, but definitive identification may still require culture, histology or molecular testing.Diagnostics & testingBacterial culture and antimicrobial susceptibility testing in fishCulture aims to recover a clinically meaningful bacterium, while susceptibility testing helps determine which antimicrobials may inhibit it. Sampling after indiscriminate antibiotic exposure can reduce the usefulness of both.Diagnostics & testingPCR testing for fish pathogens: how to interpret the resultPCR detects target genetic material, not necessarily active disease by itself. A positive or negative result has to be interpreted with sample type, timing, test validation, clinical signs and the disease being investigated.Diagnostics & testingHistopathology in fish: what tissue microscopy can answerHistopathology evaluates fixed tissues and can reveal patterns of inflammation, degeneration, parasites, neoplasia and organ injury. It is especially useful when gross appearance alone cannot separate competing diagnoses.Diagnostics & testingFish necropsy: what a post-mortem examination can revealNecropsy provides a systematic look at external and internal organs and guides what should be sampled next. It is most informative when done promptly and combined with history, water data and laboratory testing.Diagnostics & testingRadiography and ultrasound in fish diagnosisImaging can help investigate skeletal change, masses, buoyancy problems, organ enlargement or retained material without relying only on surface appearance. What can be seen depends strongly on species, size and equipment.Diagnostics & testingBlood tests in fish: when they are usefulBlood sampling can provide haematology or chemistry information in suitable fish, but reference intervals and sample volume can be limiting. It is generally a veterinarian-led tool rather than a universal aquarium test.Diagnostics & testingRecognising microscope artefacts and false positivesBubbles, debris, detached cells, food particles and environmental organisms can be mistaken for pathogens. This guide stresses movement, morphology, tissue location and repeat sampling rather than identifying a disease from a single unusual shape.Diagnostics & testingWhat a negative fish-health test result really meansA negative result may mean the target was absent, below detection, missed by sampling, altered by treatment or tested at the wrong stage. Diagnostic confidence comes from combining tests and evidence, not treating one negative result as proof of health.Diagnostics & testingPreserving, labelling and transporting fish diagnostic samplesDifferent tests need different sample handling. This guide focuses on contacting the receiving laboratory first, clear labels, chain-of-custody style records and avoiding improvised preservatives that can make a sample unusable.