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I have an AP system that is several months old, and has never cycled correctly.  I would appreciate input on some specific questions that may help diagnose the problem

- How much does pH affect cycling?  My system gravitates to low pH (6.2 ish)  Is there an ideal pH for cycling?

- I have a small (and shrinking) number of tilapia fry in the fish tank.  How high can I push the ammonia level to jump start cycling w/o killing the fish?

- Does temperature affect cycling?  I am in Virginia, and the water has been in the high 50s / low 60s fahrenheit for several months, and is now in the lows 70s

- Do I need to keep adding nitrifying bacteria until the system cycles?

The system includes three grow beds, a 180gallon fish tank and a sump.  They are all in a small greenhouse.  

Thanks, Bill

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Hi Bill,

So your system has never been cycled and you've introduced fish anyway and now they are dying one by this correct? What is the history of your system? What has been the Ammonia-Nitrites-Nitrates-temps in the past and what are they at present? What do you mean by "adding nitrifying bacteria?

For some reading in the meantime, see here

Harold - so yes, to your basic question and assumption.  I was impatient, hoped cycling was underway and added fish.  Now they are dying one by one.  

Today ammonia is 8 ppm, nitrite is negligible (maybe slightly more than zero, but not significantly), water temp is 68, though it will be up to low 70s later in the day (from prior experience).  pH today is 6.4 which has been pretty typical, though at times it has drifted lower.  I have been adding  Microbe Lift Nite Out II - Nitrifying Bacteria to start the nitrogen cycle.  The water temperature up until the last month tended to be in the mid/upper 50s.  With late spring in Virginia it is warming up now.

I set the system up in January, but did not add fish until about six weeks ago.

From your link and other reading, this is my diagnosis

- pH is too low and I need to get up around 7.2

- Temperature until recently was too low to start the nitrogen cycle

I do have an outdoor ornamental pond with a bio filter.  I have been temped to introduce some slime from that bio filter to my aquaponic system, but concerned about possible undesirable contaminants.  

Thanks for any suggestions.


Bill...trying to get nitrifying bacteria to become established in a system (i.e cycling) seems very, very difficult at such a low pH value. At pH 6.2-6.4 you will have a most difficult time. 7.6 to around 8 is about ideal. Your temps are not "too low to start the nitrogen cycle" as I had no problems cycling a system last fall/winter where water temps were in the mid/upper 50's...and even lower...

I made sure I had a nice high (8.2 or so) bacteria loving pH, and that my kH (carbonate hardness) values were well above the minimum threshold of 40ppm. (My kH was above 300). 8ppm ammonia also tends to inhibit the process. Between 1 and 4ppm is plenty.

At this point, if you're going to raise pH, it might be wise to use a carbonate/bicarbonate buffer rather than a hydroxide based one, as that would 'kill to birds with one stone'...later on once your bacteria have firmly established themselves both pH as well as kH become somewhat less important (as long as you stay within the bounds of minimum threshold values), but at this delicate stage they both probably matter quite a bit.

Hi Bill,

Your diagnosis is accurate. Vlad's right in his response. Of all the inhibiting factors I'd say the 8ppm Ammonia is the most mitigating one. You can live with a low PH and those low 70'ish temps for cycling(albeit slowish), but the high ammonia should be managed for sure. If you slowly, over a period of about 3-4 days,replace the 90 gallons from the 180 to get a 4ppm reading, this would, to no end, help your system to cycle considerably faster. Water replacement must be without chlorine/chloramines, of course.

If you want to tackle the problem more holistically, a higher PH, between 7ppm - 8ppm, and supplemental heating to around 80 F would be optimal.

Harold/Vlad - thanks for the advice.  I am raising the pH, and started swapping some water.  Will continue that tomorrow.  

What is your view of scraping a little slime out of the bio filter for my outdoor fish pond and adding it to the AP system?  


Hi Bill,

I think both Vald and I missed that question on adding the outdoor bio-slime. It is a risk doing these things but looking at the conditions involved can help to make good assessment. Has your outdoor fish pond been exposed to Antibiotics/hormone treatment? Has the fish in this system ever contracted any type of disease? Are the plants/fish in a healthy state? Are the water turnover rates in the GH system within the rules of thumb ratios or higher? Not necessary but do you have any supplemental oxygen(Aerator)?

If you introduce the bio-slime to an environment which is high in oxygen, without any stagnant pockets of water and from a safe/ish source, the risk will be very rare if any at all

Thanks Harold 

The outdoor pond has a high water flow rate, low fish density and I have not had issues with disease.  It is ornamental (a few goldfish, coy and a catfish) so no chemicals.  

I think I will give the bio-slime a try and report back.


Yeah, sorry...I didn't see that either...

What Harold said 

Just make sure to get your pH up and ammonia down so the bacteria you introduce from your pond can have a fair go at it and multiply...

Vlad/Harold - thanks for the very helpful feedback.  Here are a few status updates.

- No fish have died since Saturday when I first replaced some water in the systems (I eventually drained and replaced 20% 3 times over a period of two days)

- The water swap brought the ammonia down to about 4ppm

- I brought the pH up to about 7.6.  In my my system pH always seems to be dropping, but I will keep it around this level

- I added bio slime from my outdoor filter on Saturday

- Nitrite is finally measurably increasing - 5ppm today, highest I have ever seen it


Very cool Bill, glad to hear that things have started to take off .

Out of curiosity...what is it exactly that you are using to raise pH?

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