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Fish health decision framework
Treatment principles
Diagnosis-first treatment decisions, supportive care, compatibility and monitoring without dosing recipes.
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.
Treatment principlesSupportive care for a sick fish while the diagnosis is uncertainWhen the diagnosis is unclear, high-quality supportive care can buy time without destroying diagnostic evidence. Stable water, oxygenation, low stress and careful monitoring are often more defensible than adding multiple products at once.Treatment principlesProtecting water quality while fish are being treatedTreatment can change feeding, waste, oxygen demand and biofilter performance. This guide treats water quality as part of therapy rather than something to check only if medication appears to fail.Treatment principlesOxygenation during fish treatment: why it mattersSick fish may already have impaired gills or increased oxygen demand, and some treatments or warmer water can narrow the safety margin. Monitoring breathing and gas exchange is therefore part of treatment safety.Treatment principlesFeeding sick fish: appetite, fasting and recovery principlesFood can support recovery, but uneaten food can also worsen water quality and some diagnostic tests depend on appetite. This guide helps decide when to offer small observable feeds and when reduced feeding is safer.Treatment principlesWater changes before, during and after fish treatmentWater changes can reduce pollutants but can also alter treatment concentration or water chemistry. The correct plan depends on the product label, the diagnosis and the reason for the water change rather than a universal schedule.Treatment principlesActivated carbon and chemical filtration during medicationCarbon, resins and other media can adsorb some compounds while leaving others largely unaffected. This guide explains why the product label and treatment goal should determine whether chemical media stays online.Treatment principlesUV sterilisation, ozone and medication: when equipment can interfereUV and ozone can alter some dissolved compounds and also affect free-living pathogens. The guide helps separate the role of this equipment in system management from the chemistry of a specific treatment.Treatment principlesChoosing a hospital-tank treatment to protect the display biofilterMoving a fish to a hospital tank can isolate medication from the display biofilter and tankmates, but only if the hospital system itself has safe water and the move does not create more stress than benefit.Treatment principlesAntimicrobial stewardship for aquarium fishAntibiotics are not a substitute for diagnosis, water correction or parasite control. Responsible use means veterinary oversight where required, obtaining culture and susceptibility data when practical, and avoiding repeated empirical courses that select for resistance.Treatment principlesWhy bacterial culture can change antibiotic choiceDifferent bacteria—and different strains of the same species—may respond differently to antimicrobials. Culture and susceptibility results can narrow treatment and reduce unnecessary exposure when a bacterial infection is truly involved.Treatment principlesTreating external parasites: matching control to the life cycleExternal parasites differ in where they live and whether vulnerable stages occur on the fish or in the environment. Treatment planning should therefore be based on identification and life cycle rather than assuming one exposure treats every stage.Treatment principlesTreating internal parasites: why diagnosis and life cycle matterInternal parasites can involve the gut, tissues or intermediate hosts. A visible worm or abnormal faeces is not enough to choose a treatment route without considering where the parasite is and how reinfection occurs.Treatment principlesManaging suspected bacterial disease without shotgun medicationUlcers, fin loss, redness and lethargy can occur after environmental injury as well as bacterial infection. This guide prioritises water, lesion pattern, culture where appropriate and targeted treatment rather than stacking antibiotics.Treatment principlesManaging suspected water-mould disease without shotgun treatmentCotton-like material may represent water mould, damaged tissue, bacteria or debris. The guide focuses on confirming what is growing and correcting the injury or water conditions that allowed secondary colonisation.Treatment principlesBath, in-tank and medicated-feed treatments: choosing the routeRoute matters because a skin parasite, systemic bacterial infection and intestinal parasite are not exposed to a medicine in the same way. The guide compares decision factors without providing a dosing recipe.Treatment principlesDeciding whether to treat the display tank when invertebrates are presentA product safe for one fish may be hazardous to shrimp, snails, corals or other invertebrates. The decision is whether the whole system must be treated or affected fish can be managed separately.Treatment principlesDeciding whether to treat the display tank when sensitive plants are presentSome treatments can injure plants or disrupt planted-tank chemistry. This guide weighs hospital-tank treatment, the organism’s environmental stages and the need to treat the display itself.Treatment principlesWater chemistry and medication performance: pH, hardness and organicsWater chemistry can change drug availability, stability or toxicity, while organic matter can bind or inactivate some compounds. The product label and veterinary advice should be interpreted in the actual aquarium water, not in isolation.Treatment principlesMonitoring whether a fish treatment is actually workingImprovement should be defined before treatment starts: breathing, appetite, lesion size, mortality, behaviour or laboratory result. Without a baseline, normal day-to-day variation can be mistaken for success or failure.Treatment principlesWhen treatment is failing: stop, reassess and seek diagnosticsFailure can mean wrong diagnosis, resistance, inadequate delivery, reinfection, ongoing environmental injury or irreversible disease. Adding more medicines without identifying which explanation fits can make the next diagnostic step harder.