Bad odors from an aquarium signal a problem - healthy tanks don't smell. Fishy / sulfurous smell: dead fish or snail decomposing somewhere. Search behind plants, under rocks, in filter intake. Remove + do 30% water change. Anaerobic pockets in deep sand also smell of rotten eggs (hydrogen sulfide). Solution: gently stir top 1-2cm of substrate during water changes, or add MTS snails to bioturb. Sour / vinegar smell: excess organic decay + low oxygen. Test ammonia + nitrite. Likely indicates a stalled cycle or recent fish die-off. Do 50% water change + clean filter mechanical media. Ammonia smell: high ammonia (pungent, like cat litter). Do 50% water change immediately, dose Seachem Prime, add bottled bacteria. Earthy / fresh smell: normal. A well-balanced aquarium smells faintly like a clean stream. Saltwater normal: a salt + ocean smell, sometimes slightly briny. Bad = sulfurous. Filter cleaning: dirty mechanical media (sponges, floss) start smelling within 4-6 weeks if not rinsed. Rinse in old tank water during water changes. Lid + glass: evaporation + condensation can grow biofilm. Wipe lid + rim weekly. Smell + cloudy water = bacterial bloom + decay: reduce feeding, vacuum substrate, larger water changes for 2 weeks.
The full Q&A library answers the most-searched aquarium questions. Browse calculators, glossary, and disease database for related help.
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.
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.
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