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Understanding toxicity impacts between pH level and Ammonia

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jayc

Understanding toxicity impacts between pH level and Ammonia.

How does pH affect the toxicity of ammonia?

 

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Table 1 - the Nitrification and Ammonification process.

 

The Ammonia reading you get from test kits is actually the sum of Total Ammonia - which is made up of Ammonia NH3 + Ammonium NH4.

At low levels of pH (lower than 6.0), ammonification occurs. Remember, pH is an inverse count of Hydrogen (H). At low pH, you have more Hydrogen. At high pH, you have less. At these low levels of pH (high acidity), the ammonia NH3 'absorbs' (for lack of a better word), an extra Hydrogen ion -> becoming NH4 or ammonium.

The reason Ammonium is less toxic to fish and shrimp is because NH4 with that added Hydrogen H ion is now less permeable to the gills of fish & shrimp. NH4 is also excreted across the gills via a carrier mediated process in exchange for sodium Na+.

Ammonia toxicity is also influenced by temperature:

The lower the temperature the less toxic it becomes. Or to put it another way - NH3 toxicity increases with temperature and pH.

 
  Percent NH3 of total ammonia
Temp  pH 6.5 pH 7.0 pH 7.5 pH 8.0 pH 8.5
20C / 68F 0.13 0.40 1.24 8.82

11.2

25C / 77F 0.18 0.57 1.77 5.38

15.3

28C / 82F 0.22 0.70 2.17 6.56

18.2

30C / 86F 0.26 0.80 2.48 7.46

20.3

Table 2. Un-ionized NH3 as a percent of total ammonia (by temperature and pH).

 

Assuming a temp of 28C and a pH of 7.0 - if 5ppm of ammonia is present this results in only .03 ppm ammonia.

However, in a Tanganyikan Cichlids tank with a pH of 9.0, that has a Total Ammonia of 5 ppm, your ammonia level is 2.06 ppm! This now become toxic for the fish.

 

But, at a pH of 6.0, and 10 ppm of Total Ammonia, the ammonia is only .007 ppm. Even though we have MORE ammonia.

So be cautious when performing water changes in a low pH tank, as the low pH has an adverse affect on the nitrifying bacteria that converts ammonia to nitrite.  Because of the acidity these bacteria populations can drop so low that any change in alkalinity can cause the Total Ammonia reading to rise quickly.  While the pH stays low the Total Ammonia reading is nearly all ammonium, but if you do a water change or add an alkalinity buffer to the system, the ammonium can be quickly converted to ammonia, potentially causing ammonia poisoning.

It is good to note here that, as per the very top picture (table1) ... nitrifying bacteria that convert ammonia NH3 to Nitrate (NO3) does NOT convert Ammonium NH4 to a less toxic form. The bacteria isn't present in sufficient amounts in such low pH environments to process it. Ammonium NH4 is ever present in a low pH tank that has living creatures in it. NH4 is in there ready to be converted into NH3 at the first sign of added alkalinity during water changes. Hence, why we always tell you to match water parameters and add it into the tank slowly (drip it in if you can), don't dump in buckets of new water all at once.

So in summary, the combination of low pH (<6) and cool temperatures that the shrimp live in can mean that high ammonia levels are not toxic to them. But be careful !!! Any change in the pH buffer that increases alkalinity will cause the toxic ammonia to immediately convert from NH4 to NH3.

 

There you go. Hopefully that is a more precise explanation to aid your understanding. 

The ideal spot to be is just above 6.1 to 6.5, where bacteria still function, and ammonia is less toxic. Along with the cool temperatures some shrimp (or fish) live in,  is the best environment to be in to minimise ammonia poisoning.

I wonder how many times our shrimp die in hot temps, (say due to hot weather ... maybe even a broken heater) not because of the heat, but because our low pH tanks have not enough functioning bacteria to cope with the sudden change in toxic NH3 ammonia due to the rise in temps??? Food for thought.

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