5 Common Causes of Marine Aquarium Ammonia Spikes

Sometimes adding specimens to an already heavily-stocked aquarium will be too much for existing biological filtration

Sometimes adding specimens to an already heavily-stocked aquarium will be too much for existing biological filtration

You come home from a long, vexing day at work only to notice that the fish in your marine aquarium are all gasping at the surface of the water or cowering in a corner of the tank and behaving lethargically. Panicked, you promptly test your water parameters and discover that ammonia is present. The tank has long been cycled, so no ammonia should be detectable. What gives?

When an ammonia spike occurs in an already-cycled system, one of the following circumstances is usually the underlying cause:

#1: Overstocking

Adding “just one more” specimen to an aquarium that’s already close to being maxed out with respect to stocking capacity can easily lead to a situation in which more dissolved waste is produced than the biological filter can accommodate. It’s always better to stock lightly—even if that means the tank looks more sparse than you might prefer (the usual state of my aquariums)—than to push your luck with the bioload.

#2: Overfeeding

Excessive feeding of fish or invertebrates, which often results in uneaten food left decomposing in the system, is another surefire way to overwhelm an established system’s biofilter and cause ammonia to spike.

For most fish, it’s best to offer small, frequent feedings that can be consumed rapidly. If you keep a predatory species that requires large, meaty meals on a less-frequent basis (e.g., a grouper or moray eel), or if you offer relatively high volumes of planktonic food to invertebrates, keep in mind that water changes, filtration, and protein skimming must be sufficient to keep pace with the dissolved waste such feeding regimens typically produce.

#3: A dead specimen

A dead animal rotting in a marine aquarium can be a major source of ammonia—especially in smaller systems and with specimens of greater mass. So, whenever possible, it’s best to locate and remove deceased specimens. If removal isn’t practical, be ready to perform frequent water changes while the animal decomposes.

#4: An impaired biofilter

The loss of a portion of a system’s biofiltration capacity is a very common cause of ammonia spikes. This can happen, for example, when mechanical-filtration media (socks, sponges, etc.) are left in place long enough to become colonized with nitrifying bacteria and are then removed and replaced or aggressively rinsed.

Another possible biofilter-impairing scenario (of many) is a temporary power outage that shuts down circulation/aeration in the tank. Nitrifying bacteria require oxygenated water to thrive, so with the pumps out of commission, the dissolved oxygen level will soon begin to drop and the bacteria will begin to die off.

#5: Any combination of the above

Of course, if you can “answer yes” to more than one of the above scenarios—e.g., overstocking and overfeeding the tank, or the combination of a deceased specimen and the loss of some biofiltration capacity—the likelihood of an ammonia spike is all the greater.

What am I forgetting?
If I know me (and I do), I’m most likely forgetting something significant here. If any of you salties out there can think of other circumstances that lead to ammonia spikes in established systems, please feel free to share them in the comment section below.


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About Jeff Kurtz

Jeff Kurtz is the Co-founder/Editor of Saltwater Smarts, former Senior Consulting Editor for Tropical Fish Hobbyist Magazine, and the aquarist formerly known as “The Salt Creep.” He has been an aquarium hobbyist for over 30 years and is an avid scuba diver.


  1. Great article, as always guys. I believe you haven’t missed a beat.
    I was wondering what everyone thought about washing all your filter mediums in sump. I’ve resorted to once or twice a week washing out all mechanical filters. With DI water. Should I use DI water or clean saltwater?
    Also, Nitrogen sponge….good idea or no? Do you rinse out once a week or never? Just throw out?
    Thanks and appreciate your articles of amazing information. … !

  2. Paul Baldassano says

    I would not rinse anything in the sump as that is just returning the dirt to the system and you should never wash anything in fresh water that is used for filtration as that may kill the bacteria. Rinse it in saltwater, you can use used saltwater for that.

  3. Thank you Paul…makes alot of sense.
    And I meant rinsing the sump filters in a waste bucket (now with clean saltwater)
    Any experience with nitrogen sponge? Waste of time and destined for failure?
    I think i have an “old tank syndrome” ….10 year rock

  4. The reason I brought up the mechanical filter maintenance questions is to avoid the ammonia spikes, possibly by washing filters very regularly.
    Am I far off here?

    • Hi Jim! I usually rinse mechanical-filtration media in a bucket of salt water that I’ve just siphoned out of the tank during a water change. Sponges can be submerged in the bucket and squeezed to liberate trapped gunk. However, as I alluded in the post, you have to be careful about neglecting this chore for too long and then rinsing all your media at once.

      With respect to Nitrogen Sponge, I’ve never used it myself, so I can’t really comment on its effectiveness. Can any other salties out there chime in on this product? Generally speaking, I’m of the belief that if you have an appropriate amount of quality live rock and stock conservatively, that’s all you should need to keep the biological filter humming along.

  5. Paul Baldassano says

    Jim, about your Old Tank Syndrome and your ten year old rock, my rocks are between 30 and 40 years old and I don’t have any Old Tank Syndrome although I have Old Man Syndrome, which doesn’t seem to affect my aquarium. Old Tank Syndrome occurs if you have something like an old Deep sand bed which after about ten years stops working to help the tank and starts working against the tank. I really hate those things. I have never used a nitrogen sponge, just bath sponges. Speaking of sponges, just think how high the ocean would be if there were no sponges in it. Just a thought

    • OMG. You are hilarious…. Old Man syndrome….no sponges in the ocean……. bahahahahaaaa.
      Seriously, I so look forward to these posts! I don’t always get to them right away, but when I do….^

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