From
Fishkeeping.co.uk (relevant parts in
bold)
The second common way in which this can occur is when other factors cause the bacteria in an established filter to die off or fail to cope. This can happen for any of the following reasons:
-Something blocking the filter
-A prolonged power cut of more than a couple of hours
-Accidentally forgetting to dechlorinate tap water before using it for a water change
-A consequence of certain types of medications used to treat illnesses in the fish
-Consistent overfeeding or failing to remove uneaten food and excess waste regularly
-Death of one or more fish whose bodies are decaying somewhere in the tank
-Overstocking or introducing too many new fish at a time
The third major cause of unexpected ammonia and nitrite spikes is "old tank syndrome". This arises when tanks are not given regular partial water changes and are simply topped up instead when water evaporates.
Over time nitrates build up, carbonates deplete and the water acidifies. Testing the pH will confirm this, as "old tank syndrome" usually causes a gradual lowering of pH.
When pH drops to a low level, below pH 6, the beneficial bacteria can become dormant or even die, with the result that ammonia and nitrite begin to accumulate and fish start to suffer.
If you have very soft water with a low level of carbonate hardness (kH) this can happen quite rapidly so regular partial water changes are absolutely vital if you have this kind of water. One of the signs that a tank may be on the verge of "old tank syndrome" is when new fish are added and seem to immediately become unwell or die very soon afterwards when existing stock appear to be fine.
This is because existing fish have become gradually used to the high nitrates and low pH but the new fish introduced from healthy tanks find it too much of a shock and rapidly decline as a result.