Testing
All four readings are fine but the tank smells bad
The four standard tests measure four things, and none of them is smell. A sour, swampy or rotten odor with clean readings means organic material is decomposing somewhere the water flow does not reach — under the substrate, behind decor, or inside the filter. Find it physically; no test will point at it for you.
Act this week
Test these first
Before anything goes into the tank and before anything comes out of it, get these numbers. They cost fifteen minutes and they decide everything that follows.
- Ammonia — total ammonia — the ammonium ion plus free ammonia, added together
- Nitrate — the nitrate ion — note that drinking water standards are written as nitrate-nitrogen, which is a different number
- Temperature — the temperature of the water itself — not of the room the tank sits in
What is usually behind it
| Cause | How to confirm | Time to resolve |
|---|---|---|
| Anaerobic pockets in deep or compacted substrate | Push a finger or a vacuum tube into the bed and see whether bubbles rise and the smell sharpens. A rotten-egg note is characteristic of low-oxygen decomposition down there. | Three to four weeks working through the bed in sections |
| Something dead out of sight | Count the livestock. Check behind the filter, inside ornaments, under the heater bracket and in the intake strainer. A missing fish and a smell together settle the question. | One to two days after removal |
| Sludge inside the filter housing | Unplug the filter and open it. Accumulated fine debris under the media basket or in the bottom of the housing is common and rarely inspected. | Same day |
| Decaying plant matter and uneaten food | Dying leaves are still attached, or there is food visible on the substrate more than a couple of minutes after feeding. The smell is more musty than rotten. | Three to five days |
| A warm, still tank with poor gas exchange | The smell is worst in the evening, there is a film on the surface, and the temperature is at the top of the range for the species you keep. | Same day for the smell; one to two days for the temperature |
Anaerobic pockets in deep or compacted substrate
How to confirmPush a finger or a vacuum tube into the bed and see whether bubbles rise and the smell sharpens. A rotten-egg note is characteristic of low-oxygen decomposition down there.
What to doVacuum a section of the bed at each water change, rotating through the tank over a month. Do not turn the whole bed over in one go — releasing all of it at once is unpleasant for the tank and for the room.
Time to resolve: Three to four weeks working through the bed in sections
Something dead out of sight
How to confirmCount the livestock. Check behind the filter, inside ornaments, under the heater bracket and in the intake strainer. A missing fish and a smell together settle the question.
What to doRemove it, change 30% of the water, and vacuum the immediate area. The odor lifts within a day and the readings may never have moved at all if the filter kept up.
Time to resolve: One to two days after removal
Sludge inside the filter housing
How to confirmUnplug the filter and open it. Accumulated fine debris under the media basket or in the bottom of the housing is common and rarely inspected.
What to doRinse the mechanical media and wipe the housing out with tank water. Leave the biological media as untouched as you can and never rinse any of it under the tap.
Time to resolve: Same day
Decaying plant matter and uneaten food
How to confirmDying leaves are still attached, or there is food visible on the substrate more than a couple of minutes after feeding. The smell is more musty than rotten.
What to doTrim and remove dying leaves rather than letting them break down in place, and cut the feeding portion to what disappears in a minute.
Time to resolve: Three to five days
A warm, still tank with poor gas exchange
How to confirmThe smell is worst in the evening, there is a film on the surface, and the temperature is at the top of the range for the species you keep.
What to doRaise the filter outflow to break the surface and open the lid. Unplug the heater before doing anything else near the water. Better gas exchange changes the smell noticeably within hours.
Time to resolve: Same day for the smell; one to two days for the temperature
What not to do
These are the moves that make this particular problem worse, and they are all common. Most of them come from wanting to do something substantial when the right answer is something small.
- Do not mask the smell with a product. The material causing it is still in the tank and still decomposing.
- Do not strip the tank down and replace everything, including the filter media. That trades a smell for an ammonia spike.
- Do not turn over the entire substrate bed in one session.
- Do not conclude the tests are broken. Ammonia, nitrite, nitrate and pH are simply not smell tests, and a clean result from them is real.
When this stops being a water question
If fish are gasping, sitting on the bottom, refusing food, or showing marks or unusual coloration alongside this, that is where the water story ends. Test and change water immediately because bad water is the most common and most fixable cause — and then contact an aquatic veterinarian. This site does not diagnose fish disease and does not recommend treatments for it, and no product on this site should be used as one.
Keeping it from coming back
A tank that never smells is a tank where nothing sits still for long. Vacuum a different section of substrate at every water change so no part of the bed goes a month unattended. Open the filter housing quarterly and clean the parts that are not biological media. Trim dying leaves out rather than leaving them to break down. Feed once a day and only what is eaten. And keep the surface moving, because gas exchange is what carries the difference between a fresh tank and a stale one.
Reference for this page
Parameters
Equipment involved
- Filter Media & Cartridgeswhat actually does the filtering, and when it gets replaced
- Test Kitsknowing the number instead of guessing it
- Filtersturning water over and housing the bacteria
Related problems
Act this week
Act this week
Act this week
Published sources
- USGS Water Science School — Dissolved Oxygen and Water
- World Aquatic Veterinary Medical Association
- American Veterinary Medical Association
Published figures on this page come from the documents above. Where a value depends on species, pH or temperature, that is stated rather than averaged into a single number.