The short answer
Yellow water usually has one of two causes: tannins leaching from driftwood or leaves (a light golden tint, harmless), or a build-up of dissolved organic waste from an under-maintained tank (a duller yellow that signals it’s time for more water changes). Both clear easily — the fix is activated carbon plus regular water changes.
Telling the two causes apart
Tannins produce a clean, tea-like golden colour and come from wood or botanicals. They’re harmless and slightly acidify the water. See our fuller explainer on brown, tannin-stained water.
Dissolved organics — from fish waste, uneaten food and decaying plants — produce a murkier yellow and go hand in hand with a neglected tank. This is the version worth acting on, because it means waste is accumulating.
How to clear yellow water
- Increase water changes to 25–30% weekly, or more if organics are high. See how to do a water change.
- Run activated carbon in your filter to strip both tannins and dissolved organics.
- Vacuum the substrate with a gravel cleaner to remove trapped detritus.
- Feed less — overfeeding is the usual root cause of an organic tint.
Prevention
Consistency keeps the water clear. A weekly change and sensible feeding stop organics ever reaching the point where they tint the water. If tannins from wood are the cause and you’re happy with the look, you can simply leave it. For murky rather than tinted water, see cloudy water causes, and browse the maintenance hub for gear.