Troubleshooter

Why do my fish keep dying?

Quick answerNew tanks: ammonia poisoning (test immediately). Old tanks: declining bio-filter, accumulated nitrate, parameter drift. The #1 cause is uncycled tank or skipped quarantine introducing disease.

Possible causes (6)

1. Uncycled tank (most common)

New aquarists add fish before bio-filter establishes. Ammonia spikes kill within days. Cycle 4-6 weeks before stocking.

2. Skipped quarantine

New fish bring ich, velvet, flukes. Disease wipes out the tank. ALWAYS QT 14-30 days.

3. Old tank syndrome

Declining biofilter, accumulated nitrate, mineral drift in tanks 12+ months without water changes. Reset with weekly 25-30%.

4. Equipment failure

Heater stuck on (cooked fish), heater failed off (cold), filter dies (ammonia spike). Test equipment monthly.

5. Wrong water chemistry

pH/GH mismatch for species. Always research species parameters BEFORE buying.

6. Aggressive tankmates

Bullying causes stress + secondary infections. Watch interactions; separate aggressors.

What to do next

Test water first - many "behavior" issues are actually water-quality problems. Use the water parameter checker to score your test results, the disease symptom matcher if you observe physical signs, or the general diagnoser to narrow further. Browse the disease database if illness is suspected.

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Aquarium-keeping fundamentals

Whatever specific topic brought you here, four fundamentals govern long-term aquarium success: water quality, parameter stability, biological filtration, and species-appropriate husbandry. Skip any one and the others struggle to compensate.

Water quality: ammonia + nitrite at zero, nitrate under 30 ppm freshwater + 10 ppm reef. Test weekly with API or Salifert kits. Use our water parameter checker to score your readings against your tank type.

Parameter stability: stable wrong parameters beat fluctuating ideal parameters. Most fish tolerate a wide pH range if it's stable. Sudden swings of 0.4+ pH or 5+°F kill fish faster than chronic suboptimal values. Use temperature controllers (Inkbird) + automated dosing for consistency.

Biological filtration: the bacterial colony on your filter media + rock + substrate is the engine. Never replace all media at once. Use our filter turnover calculator to size correctly.

Species-appropriate husbandry: research adult size, territoriality, diet, and tankmate compatibility before purchase. Use our tank stocking calculator + compatibility guides.

Frequently asked questions

How long does an aquarium take to set up? 4-6 weeks for full cycling + first stocking. Use our cycle ETA calculator + how long does cycling take.

What's the best aquarium for beginners? 20-gallon long. Big enough for parameter stability, small enough for budget + space. See beginner picks.

How often should I do water changes? 25-30% weekly. See water change frequency Q&A + water change calculator.

Why does my fish keep dying? 5 leading causes: uncycled tank, wrong species pairings, no quarantine, undersized tank, neglected water-change schedule. See full diagnosis.