Dealing with Ammonia and Nitrite Spikes in Aquariums

How can the unthinkable situation of ammonia or nitrite being present in an aquarium which is already stocked with fish arise?

There are number of ways this can happen. The most common is poor advice  suggesting it’s safe to stock a new tank within days or even hours of its being set up. This is how tanks were cycled years ago when our knowledge of biological filtration and the effects of ammonia and nitrite on fish were very different to current understanding. Unfortunately, not everyone has caught up with newer findings and fishless cycling is still often not mentioned, or not recommended due to lack of up-to-date knowledge. If you and your fish are the victims of poor or outdated advice, you’ll be in a situation where the biological cycle hasn’t had time to become established, leaving fish swimming in ammonia and nitrite and at risk of pain, illness and death from their effects.

The second common way in which this can occur is when other factors cause the bacteria in an established aquarium filter to die off or fail to cope. Some ways this can happen include:

  • Something blocking the filter
  • A power cut of more than a couple of hours
  • Forgetting to dechlorinate tap water before using it for a water change
  • Use of certain types of medications used to treat illnesses in  fish
  • Consistent overfeeding of fish or failing to remove uneaten food and excess waste regularly
  • Death of one or more fish or other tank mates whose bodies are decaying somewhere in the tank
  • Overstocking or introducing too many new fish at one time

The third major cause of unexpected ammonia and nitrite spikes is “old tank syndrome”. This arises when aquariums 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 and 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 new fish introduced from healthy tanks find it too much of a shock and rapidly decline as a result.

Prevention is always better than cure and ammonia and nitrite spikes can largely be prevented by fishless cycling any new tank before adding fish, keeping stock to sensible limits, being careful not to overfeed and by carrying out regular maintenance to filters with weekly gravel vacuuming and partial water changes. Testing levels of ammonia, nitrite, nitrate and pH on a weekly basis just before the water change should alert you to any issues before they become too much of a threat.

OK, so you didn’t do a fishless cycle, or your cycle has failed and your fish are dying.  What do you do now?
  1. If nitrates are not exceptionally high, immediately do a 50% water change with dechlorinated water warmed to match the temperature in the tank and then do a 25% water change every day until your tank is cycling (i.e. ammonia and nitrite are at zero).
  2. If “old tank syndrome” is the reason for the filter crash and you have excessive nitrate levels, start by doing two or three 10% changes over a few hours on day 1 and then move to 25% daily changes when nitrates have reduced. Too large a change initially would shock the fish. Test the water daily for ammonia and nitrite until the values are holding at zero for several days running. If levels are high, do an immediate extra water change
  3. .If at all possible, get some filter media from a mature tank and put it in your filter; just cram it in along with your own media. This will introduce a small amount of beneficial bacteria which will rapidly multiply..
  4. Keep good aeration in the tank both to help the fish a little and to oxygenate those beneficial bacteria.
  5. Avoid using medications, if at all possible, until the tank is cycling. Many commonly used medications can kill off beneficial bacteria. Your fish may well be affected by ich, fungus or other infections due to the stress of the ammonia and nitrite in the water, but the immediate priority is to establish the cycle and improve the water quality.
  6. If you can temporarily rehome your fish while you carry out a fishless cycle, that is the very best option – especially for more sensitive species such as Plecos, Corys or other bottom dwellers, Tetras, Rams etc.. Try asking your local aquatics store  to look after them until your tank is cycled (after all, chances are that they got you into this mess in the first place!).
  7. Live plants can use ammonia directly, so add some cheap aquatic plants to the tank, such as Elodea or Giant Vallis.
  8. Don’t feed your fish at all if your ammonia readings are high, and only feed bare minimum rations every other day until the tank cycles. This will cut down on the ammonia the fish produce. Since fish are cold blooded creatures and don’t need the calories of a mammal they can go for several days without food with no ill effects. Your fish may not be very hungry anyway so do be careful not to feed more than the fish can eat and clean up uneaten food immediately, before it rots and produces even more ammonia.
  9. Continue to clean the gravel of obvious dirt and uneaten food as normal to prevent additional ammonia forming, but don’t aggressively clean or change any filter media. If the flow slows and the filter is blocked, just clean it enough to unblock it, using water from the tank to rinse it.
  10. Buy a bottle of Seachem Prime. This is a dechlorinator which also “locks” free ammonia into a less harmful form and provides some protection for fish from the effects of nitrite. Use a double dose sufficient to treat the whole tank every day in the 25% water change. If you use Prime, buy a Seachem Ammonia Alert so you can see the level of just the toxic ammonia – most tests also react to the “locked” non-toxic form so will not give reliable readings while treating daily with Prime or similar products.
  11. Continue this regime of daily testing, daily (or more frequent if needed), water changes, treatment with Prime or similar and reduced feeding until ammonia and nitrite are consistently reading 0. When that happens, congratulations, your tank is cycling and the fish are finally safe.

Illustration of the Nitrogen Cycle


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