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.
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