My research findings.
As you know for the past 80-years hobbyist have been led to believe that Nitrates, an end byproduct of the nitrogen cycle, is harmless to fish and only becomes a problem if Nitrates exceeds 80-100ppm.Hobbyist are more worried about high levels of Nitrates not because of the fish being bothered by it but the introduction of cyanobacteria and algae. However, through my research this cannot be further from the truth, and just because a fish shows no outward signs of stress like they do with that of ammonia poisoning or too high of Nitrites, prolong exposure too Nitrates and even at low levels does do damage.
Here is one example: A long time ago I had a Goldfish that got caught in my skimmer and was severally beaten up to the point that euthanizing the fish would have been the humane thing to do. But instead, I placed the fish back into the Anoxic-filtering Pond to see if it would recover. Well, it did, and within 3-month the fish look as though nothing had happen to it. This got me thinking why this fish survived such a traumatic experience with no intervention from me in any way, and yet why do fish, cold and tropical, seem to get parasitic infections, fungus, fins rot and so on when they appear to be healthy other whys. What unknown stresses are our animals experiencing that we as hobbyist are overlooking? Why are we overlooking these stresses?
As you know a fish’s brain is much like that of a human brain and does contain a hippocampus and fish do produce the hormones secretion glucocorticoids or (GCs for short). This has been proven in Salmon fish after going upstream to spawn, and then shortly afterword’s die off. The excess amounts of glucocorticoids adrenal hormones secretion after spawning indicates the fish have been deeply stress and already bodily functions are shutting down and being damage from the overly abundance of this stress hormone glucocorticoids. The shrinking of the hippocampus is a sure sign that too much glucocorticoid is present, and the fish is under great stress.
Like us humans when we have insomnia, most of the time 75% is caused by stress and the overly production of glucocorticoids, a stress released hormone in humans, too. Yet, insomnia will not adversely affect its victim immediately but if persistent enough it may cause other side effects such as impairing our immune system and the shrinking of the hippocampus, memory loss.
What I found is that fish under sustained exposure to Nitrates, anything over 5-ppm produce an overabundance of the hormone glucocorticoids that cause the degeneration in the fish. In all fish, the hippocampus a neutral site that is always a spot for which glucocorticoids affect showed exacerbated neuronal damage. This means that Nitrates are a stressor and can be a potential modulator of the hippocampal degeneration that I was seeing in fish exposed to Nitrates for prolong periods.
Those fish that were exposed to Nitrates of under 5-ppm or less showed very little to no excessive glucocorticoids and their hippocampus was normal. As the Nitrate levels increased along with prolong exposed the hormone glucocorticoids increased and therefore caused neuronal damage over time. This would explain why my Goldfish recovered over its stressful mishap after being placed in the Anoxic filter because Nitrates were at 5-ppm or less.
I experimented with the Carassius auratus capability to ward off stress, diseases, and parasites exposed to Nitrates of less than 5-ppm and those of long-term exposed Nitrates greater than 10-ppm going as high as 80-ppm. Those Carassius auratus that live in low Nitrate and/or free Nitrate water, like they would in nature, were less susceptible to the biological and chemical insults and had lower Glucocorticoids than those Carassius auratus that lived in stressful Nitrates laden water. The hippocampal of the later Carassius auratus showed degeneration of the hippocampal and no damage to those fish that were exposed to the low Nitrates. This would explain why Koi show a greater sheen to their skin in low Nitrate water than those in higher amounts of Nitrates. No stress! The Nitrates are the cause of higher amounts of glucocorticoids that stress the Koi unknowingly to the hobbyist.
This also may explain why my Koi and others using the Anoxic Filter have Koi over 30-years old now. It may explain why my South American Bumblebee catfish, Microsynodontis batesii in my 20-gal antique aquarium is now over 25-years old using a plenum that keep Nitrates lower than 10-ppm with two water changes a year.
Like diabetes that doesn’t kill its victims outright but will cause secondary problems like kidney malfunction becoming stage 4 or higher, heart disease, eye impairment and so on. Nitrates over time increase glucocorticoids and although not directly will kill the animal, it will eventually cause another insult to take hold and do the job instead. Once again, more research needs to be done.