Smelly Water From Your Tank? How to Fix It
Musty, eggy or metallic — each smell points at a different problem inside the tank. Diagnose by the smell itself.
Smell is the earliest reliable warning a tank gives you, and it is specific. What it smells like narrows the cause quickly.
| Smell | Most likely cause | Fix |
|---|---|---|
| Musty / earthy | Biofilm and algae on walls | Jet wash + disinfection |
| Rotten egg | Sulphate-reducing bacteria in stagnant sediment | Full desludge + chlorination |
| Metallic | Iron or corroding pipework | Tank clean; check GI pipe run |
| Chlorine-heavy | Over-dosed bleaching powder from a previous clean | Flush and refill; use dosed disinfection next time |
| Sewage-like | Cross-contamination or seepage into a sump | Urgent — stop use and get the sump inspected |
The eggy smell is the one to act on
Hydrogen sulphide from sulphate-reducing bacteria means there is a stagnant, oxygen-poor sediment layer at the base of the tank that has been undisturbed long enough for anaerobic bacteria to establish. It will not clear with a rinse and it will get worse. That tank needs full extraction, jetting and chlorination.
The sewage smell is an emergency
If water from your tank smells of sewage, stop drinking and cooking with it immediately and switch to bottled water. In a sump this usually means seepage through a crack or a cross-connection with a drainage line. That is a contamination event, not a cleaning schedule problem, and it needs inspection before it needs cleaning.
Why the smell comes back
Because the biofilm was never removed. Biofilm is a bonded bacterial layer that a brush smears and a rinse does not touch. It regrows fast from what is left behind, which is why a manually cleaned tank often smells fine for ten days and then smells again. Pressure jetting is what actually removes it.
What to check yourself first
- Is the lid cracked, missing or propped open? Insects, birds and dust have a route in
- Is there an overflow pipe without a mesh? Same problem
- Is the tank translucent and in direct sun? That is an algae generator
- When was it last cleaned — genuinely cleaned, with sludge removed?
Frequently Asked Questions
- Can I just add bleaching powder to kill the smell?
- It masks it for a few days. The sediment and biofilm producing the smell are still there, and unmeasured bleaching powder makes the water unpleasant in its own right.
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