Why Won't My Fish Eat?

Feeding refusal is the aquarium hobby's most over-interpreted symptom. A fish that ignores food for 24 hours after transport is behaving normally; one that has not eaten for three weeks and is visibly hollow-bellied is telling you something is wrong. The critical first step is not guessing the cause — it is establishing the timeline and noting every accompanying sign, because the causes of acute refusal (one to three days) and chronic refusal (more than a week) overlap only at the margins.

Part of the Complete Aquarium Care Guide.

Main Causes

Ten distinct causes account for the vast majority of feeding refusal in freshwater aquaria, sorted here by typical onset and urgency.

How to Identify the Problem

Work through these questions in order. The answers narrow the differential quickly without requiring guesswork.

Timeline first

Day 1–3 after arrival or a major disturbance (large water change, rescape, temperature shift): assume stress as the primary cause. Review the Acclimating New Fish protocol for transport recovery. Do not attempt to force-feed. Offer food once daily and remove uneaten portions after two minutes.

Day 4–7 with no improvement: run a full water-quality test — ammonia, nitrite, nitrate, pH, KH, and temperature. Refusal that persists beyond three days in an otherwise stable tank is rarely pure stress. Even a modest ammonia reading of 0.25 mg/L is enough to suppress appetite in sensitive species.

Beyond one week: treat as a disease or chronic stressor until proved otherwise. Observe faeces colour and consistency, body shape, and resting posture. Hollow belly alongside stringy white faeces in a cichlid is a strong pointer towards Hexamita or Spironucleus infection (Noga, 2010). A fish that refuses food, remains clamped, and is developing external lesions needs isolation — see Bacterial vs Fungal Disease for differential diagnosis, and Quarantine Tank Protocol for isolation procedure.

Species-specific baselines

Some species have feeding baselines that look alarming to aquarists unfamiliar with them.

Otocinclus (Otocinclus spp.) are high-risk in this regard. Their metabolic reserve is small, and in a tank without substantial biofilm — common in new setups or recently bleached aquaria — they can starve within days even in the absence of disease. If otocinclus are not eating, the first question is whether the tank has been running long enough to develop aufwuchs on glass and hard surfaces.

Discus (Symphysodon spp.) go off feed rapidly when temperature drops below 27 °C. These fish are distributed across Amazonian blackwater tributaries where water temperatures rarely fall below 28 °C, and their physiology is calibrated to that range. A feeding strike in discus should prompt a thermometer check before anything else. A drop from 29 °C to 25 °C — straightforward in an unheated room during winter — is sufficient to produce complete feeding refusal.

Loricariid catfish commonly fast for one to several weeks post-import. A bristlenose or common pleco sitting motionless on driftwood and ignoring food for the first week is not a sick fish. It is a stressed nocturnal grazer adjusting to a new environment.

Accompanying signs

Risk and Severity

Short-term feeding refusal (up to one week) carries low mortality risk in healthy adult fish of most species. The exceptions are small species with limited body reserves — otocinclus, pygmy corydoras, ember tetras — where a week without food in a bare or immature tank can cause actual starvation.

Chronic refusal beyond two weeks is a more serious sign regardless of species. Progressive weight loss accompanies diseases that are either treatable only in their early stages (internal flagellates, certain bacterial infections) or not reliably treatable at all (mycobacteriosis). Acting at week two rather than week six makes a meaningful difference to outcome.

Water-quality triggers are the most immediately dangerous category. A fish refusing food in a tank reading 1 mg/L ammonia does not have a feeding problem — it has an ammonia problem. Correct the parameter first.

Solutions and Actions

Test water before doing anything else

Ammonia, nitrite, nitrate, pH, KH, and temperature. No food variety or behavioural intervention resolves feeding refusal rooted in poor water quality. If ammonia or nitrite is detectable, perform a 30–50% water change with temperature-matched, dechlorinated water and re-test after 4–6 hours.

Food trial

Offer the correct food for the species: sinking pellets for benthic feeders, frozen bloodworm or daphnia for reluctant carnivores, blanched courgette placed in the dark for plecos and other loricariids. Species-specific mismatches are common with newly acquired fish. Predatory mid-water ambush fish — such as glass catfish (Kryptopterus vitreolus) — will often ignore flake that floats well above their natural strike zone.

Offer food at the correct time of day. Most plecos, synodontis catfish, and kuhli loaches are nocturnal; food dropped under bright lights at midday will be ignored by species that are simply not active yet.

Rule out bullying

Watch the tank through a full feeding event without disturbing it. If one or two fish dominate the feeding zone and subordinates cannot approach, refusal in those individuals is a spatial problem, not a health one. Feed at multiple points simultaneously, add visual breaks with plants or rockwork, or consider temporary separation of aggressive individuals.

Treat if disease is indicated

Internal parasites (Hexamita, Spironucleus): metronidazole is the standard treatment. It can be administered by soaking dried food (200–250 mg metronidazole per 100 g food, fed over five to seven days) or as a bath treatment in a quarantine tank at the manufacturer's recommended dose. Treat the entire tank in confirmed cichlid hex outbreaks — subclinical infections are common in apparently healthy tankmates.

Bacterial infection with external signs: isolate the affected fish. Assess whether the presentation suggests gram-positive or gram-negative infection to guide antibiotic selection, or consult a veterinarian with fish-medicine experience. Treating a full display tank with broad-spectrum antibiotics damages the biofilter; always treat in isolation where possible.

Prevention

• Research the diet before buying. Otocinclus starve in immature tanks not because they are inherently fragile but because their food source — biofilm and soft algae — is absent. Know what a species actually eats and confirm the tank can supply it before purchase.
• Quarantine all new arrivals. Feeding refusal triggered by internal parasites or bacterial infection in a new fish exposes every established fish in the tank. A four-week quarantine provides time to observe and treat before introduction. Full protocol at Quarantine Tank Protocol.
• Maintain stable parameters. Temperature swings of even 2–3 °C over 24 hours suppress appetite in warm-water specialists. A reliable heater verified by an independent thermometer is not optional for discus, rams, or other thermally sensitive species.
• Avoid chronic overfeeding. A colony fed to visible satiation three times daily will eventually display widespread feeding disinterest, elevated nitrate, and increased bacterial load — conditions that compound one another.

Common Mistakes

1. Buying more interesting food before testing the water. The first response to feeding refusal should be a water test, not a shopping trip. Live or frozen food will not be eaten in a tank reading 1 mg/L ammonia.
2. Assuming all feeding refusal is stress and waiting indefinitely. Stress-related refusal resolves in three to five days. Anything beyond a week needs active investigation.
3. Medicating without a diagnosis. Metronidazole, antibiotics, and antiparasitic treatments each target different pathogens. Reaching for the wrong one wastes time, stresses fish, and damages the biofilter.
4. Feeding during the dark period and concluding the fish has no appetite. Diurnal species do not feed at night. Many catfish and loaches show the opposite pattern. Observe when the fish is actually active before drawing any conclusions.
5. Ignoring body condition. A fish that appears to approach food but is losing weight over weeks likely carries an internal parasite load that outpaces caloric intake. Photograph suspects weekly at a consistent angle — gradual hollowing of the belly behind the pectoral fins is often visible well before it becomes obvious to casual observation.