Why is My Tank Re-cycling?

[Leeny]

I don't know if anyone will be able to answer this, but I have a 29 gallon tank. It's been up and running for 4 to 5 years. I do frequent water changes, have never removed the gravel, and don't change all of the filter inserts at once. All of a sudden my nitrates are at zero, my ammonia is high, and nitrites are starting to climb! Can anyone tell me why this is happening?

 

[fort384]

hmmm, sounds like something killed off your biofilter. Have you added anything to your tank lately? New drift wood, new decoration, medicine for fish, anything cleaned with bleach, forget to use dechlorinator, etc etc?

 

[timwag2001]

what have you done to your tank recently, within the last week or so?

 

[bs6749]

Your nitrates won't be zero unless you have live plants/excessive algae, or did a 100% water change or so many water changes that it would be the equivalent of a 100% change.

 

[Leeny]

The only thing in the last week that I did was a 1/3 water change. I've been scrubbing a lot of algae lately, could that be it?

 

[bs6749]

My theory of what is going on in your tank is that your "regular water changes" weren't regular enough. This allowed ammonia, NH3, to be converted to ammonium ion, NH4+, which is not harmful to fish. This happens in established aquariums, that don't get regular water changes. Things break down and decay and the pH drops so the harmful NH3 becomes ammonium ion. The ammonium ion will be converted back into ammonia when a large water change is done because the water from your tap is most likely alkaline. Thus, you have an increase in ammonia that the bacteria can't handle and when they do it is converted into nitrite after they consume the ammonia. Now, you don't have any nitrates yet and this is either because you have sufficient algae to take care of it or because the other bacteria hasn't had sufficient time to convert the nitrites into nitrates. What little nitrates they ARE producing are being consumed by the algae. My guess is that you may see an increase in nitrates in 5 or so days.

 

[timwag2001]

so the test kit doesnt test both forms of ammonia?

 

[jsoong]

The Nessler kits will detect both NH3 & NH4, the Salicylate kits only NH3.

BS, I am not sure I follow you. You are postulating that bacteria cannot metabolize NH4, but only NH3?? Although it is true that only un-ionized particles can cross the cell membrane without active transport, it does not follow that the bacteria cannot take in NH4+.

The proportion of NH3 to NH4 is fixed based on pH.
NH3 + H+ <-> NH4+
This reaction is reversible & governed by the Ka of ammonia.

As the NH3 is taken up by the bacteria, some of the NH4 is going to be converted to NH3 (to maintain this fixed ratio), which in turn gets taking up by bacteria. The end effect is that the bacteria will (& do) consume NH4. <This is the main mechanism why ammonia bounded by Prime or other dechlor is still available to the biofilter & will be metabolized. To proof this, measure the ammonia level with a Nessler test after you dechlorinated water with chloramines .... The measured NH4 will drop to zero in a mature tank after a few hours.>

So, no, I don't think this has anything to do with changing pH or buffering capacity. Something is killing off the biofilter & the tank is experincing a mini-cycle. <That something could be very low pH ... say 5 or less .... but I would expect that fish would be stressed if not killed with that kind of pH crash. More likely cause would be soap, chlorine, meds or other things introduced into the tank.>

 

[bs6749]

BS, I am not sure I follow you. You are postulating that bacteria cannot metabolize NH4, but only NH3?? Although it is true that only un-ionized particles can cross the cell membrane without active transport, it does not follow that the bacteria cannot take in NH4+.

The proportion of NH3 to NH4 is fixed based on pH.
NH3 + H+ <-> NH4+
This reaction is reversible & governed by the Ka of ammonia.

As the NH3 is taken up by the bacteria, some of the NH4 is going to be converted to NH3 (to maintain this fixed ratio), which in turn gets taking up by bacteria. The end effect is that the bacteria will (& do) consume NH4. <This is the main mechanism why ammonia bounded by Prime or other dechlor is still available to the biofilter & will be metabolized. To proof this, measure the ammonia level with a Nessler test after you dechlorinated water with chloramines .... The measured NH4 will drop to zero in a mature tank after a few hours.>

So, no, I don't think this has anything to do with changing pH or buffering capacity. Something is killing off the biofilter & the tank is experincing a mini-cycle. <That something could be very low pH ... say 5 or less .... but I would expect that fish would be stressed if not killed with that kind of pH crash. More likely cause would be soap, chlorine, meds or other things introduced into the tank.>

I'm saying that this is a case of "old tank syndrome", where the pH has gone acidic, causing some of the ammonia present to convert to ammonium. I completely agree that bacteria can use ammonium ion, but if ammonia is present first it will be used more greatly than the ammonium ion. It wouldn't make sense for them to spend energy when it's not needed, therefore the ammonia would be consumed first.

Now, since there is less ammonia present, the bacteria population will start to decline and the bacteria will be spending resources on active transport and the rate of reproduction will be decreased. This leads to a slow accumulation of ammonium ion in the tank and a gradual decrease in pH, which if too low will start to kill off the bacteria, further leading to an increase of ammonia (since less will be consumed). A larger percentage of this ammonia will be converted to ammonium ion since the pH is decreasing, further progressing this cylce.

So, what you have essentially is a disaster waiting to happen. Amateurs and those that aren't too familiar with the chemical processes involved may think that the best thing to do for their tank is to change a large portion of the water since they haven't changed it in a while. This large influx of fresh water raises the pH, causes the ammonia and ammonium equilibrium to shift to that of ammonia under these alkaline conditions, and since the beneficial bacteria aren't present in sufficient numbers you start to see a mini cycle. Ammonia is converted to nitrites and then to nitrates. Since there aren't any nitrates present that indicates a false reading or that there is sufficient algae present to consume the nitrates.

I just saw that there are plants present so that explains the 0 nitrates and also indicates a nutrient imbalance because algae is present in a planted tank.

 

[gzeiger]

Do you have any basis for saying that, or is it just speculation?

I don't believe that an acidic environment (within the tolerance of tropical fish) or even an environment with another oxidizing agent that would maintain essentially 100% ammonium, would starve out the bacteria. If there is food available their population will expand to consume it.

 

[bs6749]

I'm not saying that it has all ammonium, just that the equilibrium reaction shifts so that more ammouium is present at a lower pH.

I just saw that there are plants in the tank so this means that there most likely won't be much ammonium since the plants would use it as well. Also, there is an imbalance with the nutrients and amount of lighting since algae is present in the tank.

It may still be possible that ammonium accumulated in the tank over several years and that the plants didn't use it because they didn't have sufficient lighting or because they are slow growing plants or had other forms of nitrogen present. A large PWC with alkaline water still could explain the increase in ammonia and the resulting nitrites.

 

[gzeiger]

I don't believe for a minute that an acidic pH will stunt the bacteria population to the point where they metabolize less than all available food. Algae are also very efficient at ammonia uptake and will take care of the situation if it was building for more than a few weeks.

Old tank syndrome as I understand it usually refers to the accumulation of waste in the gravel which can decompose suddenly when disturbed and exposed to oxygen, resulting in a sudden large input of ammonia far beyond the capacity of the filter. Could that have happened?

 

[jsoong]

I agree with you gzeiger. It does not matter if most of the ammonia is in ionic form ... some WILL be un-ionized. The bacteria will remove all the NH3, and that will cause a shift of NH4 to NH3, producing more NH3 for the bascteria to consume. This is a physical process & requires no energy (or any action) from the bacteria. So I would expect the bacteria to grow to matabolize ALL the ammonia, regardless of form (NH3 or NH4). It is simply not physically possible to accumulate NH4 (from pH or other binders) & still not have NH3 present.

Old tank syndrome is usually associated with high nitrates. If the plants are able to reduce nitrates to zero, they should be able to take up an ammonia input to prevent that spike. So I think that is a less likely cause of the spike.

 

[bs6749]

It is simply not physically possible to accumulate NH4 (from pH or other binders) & still not have NH3 present.

Actually, it IS possible to have nearly ALL ammonium present and no ammonia. This happens when the solution is acidic, such as an established tank. The more acidic the solution, the more ammonium ions present and the fewer the ammonia ions present. At room temperature there are about 1000 NH4+ ions for every 1 NH3 molecules in a neutral solution. When the pH falls down to 5.0 or so, which is very feasible in an established aquarium, the ratio can become more like 100,000 NH4+ ions to every 1 NH3 molecule. This means that the "food" for these bacteria is going to become less dense and if you remember from biology class, this in turn means that the population of the bacteria will become less dense as well.

So say that this process has already happened, the pH has dropped over time, the amount of mitrosomonas and nitrobacter has slowly declined and you never see an increase in ammonia nor nitrites because the bacteria population, though declining, is still able to use all of the available "food". There are still both types of bacteria present and it fully handles the load of the tank at this more acidic pH. Whatever isn't used by them is in the form of NH4+ and when they use up all of the NH3, more NH4+ will certainly be converted to NH3. Just keep in mind that it now takes roughly 100,000 molecules of NH3 to be used up before a single NH4+ molecule will spontaneously be converted. This means that the NH4+ is still increasing overall in the tank as the bacteria aren't able to consume it at a rate faster than NH4+ is being produced by the fish. No matter to the fish, they aren't harmed by the ammonium ion.

So now, you add fresh water to the tank and there is a huge change in the equilibrium and there are far more NH3 molecules present than the bacteria can consume. Now the population grows to catch up and to reproduce. They produce more nitrites in the tank than were present at the lower pH, so another bacteria population must now catch up and use up the nitrites. This is exactly why there has been a mini cycle in the tank. No nitrates present because of all of the plants and algae that are consuming them. The owner of the tank might not even see any nitrates through all of this.

 

[gzeiger]

Research shows that to be correct. Interesting. For the curious: Ammonia oxidation in Nitrosomonas at NH3 concentrations near Km : effects of pH and temperature

I really thought they could metabolize ammonium better than that.