Effects of Ammonia on Aquarium Fish

Pleco Catfish Hypostomus Plecostomus aquarium fish

Ammonia buildup results from a breakdown of fish metabolism. Since ammonia (NH3) constantly converts to ammonium (NH4+) and vice versa, ammonia test kits usually measure both, resulting in a total ammonia (ammonia-N) concentration.

In an established tank, the reading of this test needs to show an undetectable level at all times. A detectable presence of total ammonia requires immediate action.

Ammonia is highly toxic in freshwater tanks, but even more toxic in reef and saltwater environments. This is due to a higher pH level that causes the presence of ammonia gas, which in turn is far more toxic and water soluble.

Even low concentrations of ammonia-N severely stress fish; this makes them vulnerable to diseases, thereby shorting their life span. Accumulating ammonia will not only be highly toxic and cause severe stress to the fish, but will be lethal!

Ammonia toxicity is significantly influenced by temperature and pH. The lower the temperature and pH, the more ammonia can be tolerated. For example, a fish tank kept at 68 degrees F can tolerate more than twice the amount of ammonia as a tank kept at 86 degrees F.

Lethal ammonia concentrations at a pH of 6.5 are 0.73 ppm, while at pH 8.5, only 0.17 ppm is considered lethal to inhabitants.

Leave a Reply

2 Comment threads
1 Thread replies
Most reacted comment
Hottest comment thread
3 Comment authors
Notify of

THANK YOU for the question as well as the solution! 🙂


My tank ammonia is spiking to 8ppm. I did a 25% water change for 5 days and lowered the ammonia to 2ppm then I got a bloom so I stopped doing water changes. It has been 3 days of the bloom and now the ammonia has spiked back up to 8ppm. I don’t know what to do. I am trying to be patient. I lowered the tank to 78 because I read the fish could tolerate the ammonia easier at a lower temp. It is a fresh water 55 gallon tank. I checked for dead or decay and found none.

Scott @ Algone

There are a lot of reasons for ammonia to spike, but all of them indicate that the biological filter (nitrogen cycle) is out of balance. The only course of action to correct the problem is to limit the amount of organics going into the tank. Checking for dead or decay was a good first step. Next you’ll want to make sure you don’t have too many fish in the tank. A general rule of thumb would be about 1 inch of fish for every 2 gallons of water. Overstocking will produce excess waste and strain the biological balance of the… Read more »

Click to Hide Advanced Floating Content
Cart Menu Button Image0
Your Cart