Well, I tried to make it short. lol
Nitrates are a measure of pollution. They indicate that things have gone bad but they're not necessarily the cause but rather the effect.
Old tank syndrome is a scenario associated with overstocking, overfeeding, dirty clogged filters, dirty tank, low oxygen in both water and filter. I think it is incorrectly called "old tank" syndrome because it can happen in any tank, new or old.
From the perspective of the fish keeper, what one may notice is chronically sickly fish, random fish deaths, white bacterial blooms, the KH will be going down, the GH, TDS and nitrates will be rising, the pH will drop and may crash. But neither of these are the actual cause. They're just measures of the effect of the microbial shift in composition.
In simple terms, the system can't cope with the bioload anymore.
After a prolonged period of elevated nitrification levels and heavy oxygen decomposition due to the higher bio-load, the nitrate rises, the oxygen levels drops which in turn leads to the good BB suffering a blow and being overwhelmed by faculative anaerobic heterotrophic community of bacteria. This type of bacteria, unlike the good nitrifiers, can use all sorts of things for energy, both chemical(e.g. ammonia,nitrite, nitrate) and organic, and can multiply in both aerobic and anaerobic conditions, shifting their mechasims and energy sources based on the conditions. The good nitrifyers on another hand are chemo autotrophs that require oxygen to survive. Which means that if your filter gets clogged, the nitrifiers take a blow.
Heterotrophs, although having ability to assimilate ammonia in aerobic environment, are very poor at that, leading to deteriorating water conditions. When the oxygen levels drop too low, they start anaerobic decomposition and denitrification. They can use nitrate as a source and produce back hydrogen sulfide, methane, ammonia and nitrite, etc.....That's when one may start noticing some spikes.
In studies done on RAS systems on toxicity of nitrates to fish, any study that measured nitrites alongside nitrates, reports increase in nitrites when high nitrates are present.So it is important to note that these two parameters are very closely correlated and when nitrate is elevated, so are nitrites.
The studies speculate that the rise in nitrites in systems with high nitrates is due to increased nitrification levels(heterotrophs use nitrates for denitrification in low oxygen conditions). The studies also point out that the danger is magnified in low flow through systems(systems with low flow through the filters).
On a physiological levels in fish, nitrate and nitrite toxicity have the same symptoms, only that nitrites are way more toxic at massively lower levels. So what one sees and associates with nitrate toxicity in their fish tank can actually be a nitrite poisoning going unnoticed. As long as there is some nitrifiers and oxygen left to cope with the increased nitrites, they'll oxydize them back into nitrates but the cycle continues until one addresses the main problem.....
On a side note, nitrites are less toxic in harder water, ammonia is more toxic in harder water. Denitrification in marine tanks may be ok, due to the low toxicity of nitrites in salt water but in fresh water it can be disastrous. I still raise my brows when I hear fresh water denitrification..
Would I be correct in saying that our nitrobacter can't live in a ph of less than 6, or thereabouts?
Nitrification happens at all pH levels, it is just that different organisms are involved . A sudden shift in pH can indeed give the types established in ones tank a shock but this depends on the diversity of microbial community. Planted tanks for example are way more resilient because they support very diverse and complex microbial communities and can also cope with rise in nitrogenous waste when the filters have taken a hit for one or another reason.
In technical terms, a pH drop is due to the buffering capacity being exhausted(KH drop). KH is used up in nitrification and is ever dropping in a tank. The more the bio-load, the less the water changes, the softer the water, the faster it will drop, and so will the pH as a consequence of events.
