I recently posted an article concerning maintenance and care of an indoor aquaponics system in the winter. Now, I'd like to provide an update.
It is currently the 16th of January, 2020. Over the last few weeks I have followed the information provided in my last article, and closely monitored the tank temperatures and fish behavior. During the Christmas/ New Year holiday, there were a few particularly colder days in which the temperature of some of the fish tanks dropped, specifically that of tank 4 (image 1). Slightly concerned, I made sure to see whether the fish were still responding enthusiastically to feeding and eating all the food. They were. There were no food leftovers 30 minutes and an hour after feeding, and behavioral responses were continuously recorded as 4 or 5. As a result, I concluded that the temperature drop had no detectable negative effect on fish metabolism and enough ammonium was entering the system.
Image 1 - Tank temperatures (top photo dates 28/12/2019; bottom photo dates 30/12/2019)
(Low) Temperatures, however, cannot only affect the fish but also the nitrifying bacteria. Nitrifying bacteria are sensitive to changes and prefer environmental stability. Drastic alterations in temperatures can affect their ability to effectively convert ammonia. Levels of nitrate and nitrite were regularly measured and remained at appropriate levels. It was therefore assumed that temperature was not affecting the nitrification process efficiency.
Temperatures affect the functions of fish and bacteria in our aquaponics system. In turn, these affect the aquaponics nitrogen cycle, and hence pH. The pH level is measured every day, because even slight alterations can have detrimental consequences. We want to aim for a pH of about 6 for a healthy system. If too acidic, we add bases, and vice versa.
The pH level over the winter weeks has been consistently low at about 4.8 to 5.5, even with the addition of potassium hydroxide (KOH). Given the chemical equation of nitrification, we may expect a low pH to be an indication of high levels of nitrates. However, the bacteria responsible for this conversion function much slower at a lower pH. It is possible for nitrifying bacteria to adjust to such low levels, but adjustment takes time. You can maintain good nitrification at lower pH values the older the aquaponics system with a continuous low pH is. Since our system is about 8 years old, the bacteria should be adjusted and therefore nitrification should be fine.
After this deliberation, I concluded that the low pH, as long as it remains above 4.5, is not negatively affecting nitrifying bacteria. However, two days ago I did some water tests and the ammonia/um test result was higher than I have personally ever recorded. The measurement was 0.5ppm while we want to keep a level below 0.1ppm.
What to do with an ammonia spike? Reduce fish feeding immediately to reduce the amount of nitrogen entering the system. Also, do not feed tanks with potentially dead fish for a few days. Yet, we do not have any dead fish and the fish are still responding very enthusiastically to feeding. Therefore, so far, I have not reduced the amount of feed provided. Next week I will do the water tests again and see whether there are any changes in the recording.
It is vital that bacterial nitrification is at high efficiency because we want to avoid ammonia poisoning. To better understand this, I will briefly explain ammonia/um and pH. Ammonia (NH3) is toxic to fish and plants, while ammonium (NH4+), the ionized version of ammonia, can easily move around in fish. Ammonia tends to predominate when pH is high (basic) whereas ammonium predominates at low pH levels (see graph 1). So, we want a pH slightly acidic (below 7) so that ammonium is dominant in our system, ideally at around pH 6. The problem is that at low pH nitrification is a lot slower. Yet, as mentioned, these bacteria can adjust, and therefore in our relatively older system, this is likely not an issue. Higher pH can also increase nitrification efficiency, but this would mean risking high ammonia levels.
Graph 1 - Rough sketched graph showing ammonium predominates at low pH and ammonia at high pH
A low pH, low tank temperature, and potentially high level of ammonia, are all possible risks involved during the colder winter months, as metabolisms and reactions in fish and bacteria can be affected. What's more, it's clear how interconnected the various processes, nutrients, and agents in our aquaponics system are. A change in one thing could create changes or affect another agent down a reaction chain. Therefore a close monitoring of this fragile system is paramount.