Yellow aquarium water indicates dissolved organic compounds (DOCs) accumulating in the water column. Most common: driftwood tannins. Mopani, malaysian, and spider wood release tannins for 3-12 months. Tannins lower pH slightly + create a "blackwater" look that some fish (bettas, apistos, tetras) prefer. Not harmful, just visual. Soak/boil new wood before adding to reduce. Indian almond / catappa leaves: intentional tannin sources for shrimp + betta tanks. Tea-stained look is by design. Decaying food / fish waste: leftover food breaking down into DOCs. Symptoms: yellow tint + film on surface + slight sour smell. Fix: reduce feeding, clean filter, vacuum substrate. Aging tank without water changes: DOCs accumulate over time. Do a 40-50% water change immediately, then return to 25% weekly schedule. Removal methods: activated carbon (rinse + run for 3-4 weeks, replace), Seachem Purigen (regenerable with bleach + dechlorinator), large water changes. Saltwater specifics: protein skimmer removes DOCs before they yellow water. Yellow water in a reef tank usually means undersized or broken skimmer. When yellow IS bad: if pH dropped + fish are gasping + smell is sour - that's anaerobic decay, not tannin. Test water immediately and gravel-vacuum.
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