Plant filtration and Nitrates

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The most effective plants are those not limited by either CO2, light or nutrients...Underwater plants, if no co2 is injected, are always CO2 limited.....Plants with unlimited aerial CO2 and sufficient light will be nutrient limited early or later in a fish tank if not supplemented....

My coworker with the tank I tested was asserting that he only needed water changes every few months because his plants keep the nitrates low.

Lol, I must have missed the part where you mentioned he does not do water changes......There is nothing one can do to avoid water changes I am afraid....for thousands of reasons and nitrates are just one of them.

What bothers me is that they are often touted as a cure-all for reducing nitrogenous waste.

Is denitrification a cure all for nitrogenous waste?
The necessity of denitrification in fish tanks is questionable, because one needs to aim at removing nitrogen at source(in the form of ammonia, fish poop, detritus and other organic compounds), not after it has gone through several chemical and bacterial processes which are highly oxygen demanding and produce other toxic waste which we do not know how to test for....Denitrification does not add oxygen to the water. Plants do. Oxygen is the limiting factor of a healthy aquatic system...and any method not taking this into account will end up in disasters.

Denitrification requires the opposite of what one should aim in resh water tanks, i.e. anoxic conditions....Do you think that maintaining an anoxic zone in a closed system will not have side effects such as promoting the growth of wrong type of bacteria and organics decomposition that produces toxic substances, or other pathogenic bacteria?

Planted tanks have a giant nitrification ability due to their delivery of oxygen in the water column and the substrate, around their roots.....You cannot achieve that amount of oxygen in the water column and especially not in the substrate by mere surface movement....They additionally remove fixed nitrogen in all forms and that's a known fact. They remove other toxic substances, heavy metals, etc and play a role in creating the right type of microbial system favourable to fish. They may not remove all nitrogen or remove all possible toxic substances created in a fish tank, one needs to do water changes...but a planted tank is a healthier eco-system than unplanted tanks...How is denitrification a better alternative?
 
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The most effective plants are those not limited by either CO2, light or nutrients...Underwater plants, if no co2 is injected, are always CO2 limited.....Plants with unlimited aerial CO2 and sufficient light will be nutrient limited early or later in a fish tank if not supplemented....

I get what you're saying and it's true with exceptions such as macrophytes that can fix CO3. Also, it's worth pointing out that many (most?) plants are limited by the CO2 in the atmosphere of this planet. Even terrestrial plants grow faster with the current levels of CO2 than they have in the past. Finally, if there is no limiting nutrient, organisms are limited by their growth rates. But yes I agree that plants grow faster when their needs are met!

Is denitrification a cure all for nitrogenous waste?
The necessity of denitrification in fish tanks is questionable, because one needs to aim at removing nitrogen at source(in the form of ammonia, fish poop, detritus and other organic compounds), not after it has gone through several chemical and bacterial processes which are highly oxygen demanding and produce other toxic waste which we do not know how to test for....Denitrification does not add oxygen to the water. Plants do. Oxygen is the limiting factor of a healthy aquatic system...and any method not taking this into account will end up in disasters.

Denitrification, when combined with nitrification (which all tanks should have), will effectively remove ammonia, nitrite, and nitrate. It directly removes nitrite and nitrate. It does not add oxygen to the water, but it does add carbonate (which is removed during nitrification) and can help maintain pH. Oxygen should NEVER be a limiting factor in home aquariums. If it is, then then I would hardly call the system "healthy". It is limiting in intensive aquaculture (we must run liquid oxygen at all times at work) but most aquarium setups will not come close to harmful concentrations. I measure dissolved oxygen in about 70 tanks per day, and even an "overstocked" aquarium shouldn't run into issues with the filters and air pumps available on the market. We use pure oxygen because we deal with stocking densities up to 100kg per cubic meter.

Denitrification requires the opposite of what one should aim in resh water tanks, i.e. anoxic conditions....Do you think that maintaining an anoxic zone in a closed system will not have side effects such as promoting the growth of wrong type of bacteria and organics decomposition that produces toxic substances, or other pathogenic bacteria?

Anoxic zones are part of natural aquatic ecosystems. A balanced ecosystem will promote a healthier aquarium. The notable danger of these zones occurs when you have an abundance of bioavailable carbon and a lack of nitrite/nitrate. This can happen when you have lots of denitrification and a very dirty tank. Then it's possible that hydrogen sulfide is produced, which can be toxic in very high levels. Fortunately you can smell it in very low concentrations. Sometimes you see this in tanks with sandy substrates, and its presence can be seen as black patches deep in the sand. If you see that then you have too many organics and almost no nitrate. I saw it once in a tank with play sand, but no issues in the fish or their fry was seen as there wasn't much and the tank had an aerator.

Planted tanks have a giant nitrification ability due to their delivery of oxygen in the water column and the substrate, around their roots.....You cannot achieve that amount of oxygen in the water column and especially not in the substrate by mere surface movement....They additionally remove fixed nitrogen in all forms and that's a known fact. They remove other toxic substances, heavy metals, etc and play a role in creating the right type of microbial system favourable to fish. They may not remove all nitrogen or remove all possible toxic substances created in a fish tank, one needs to do water changes...but a planted tank is a healthier eco-system than unplanted tanks...How is denitrification a better alternative?

I agree that plants make for a healthy ecosystem. As stated, I believe in plants. I keep them in all of my freshwater tanks. I just believe that anoxic zones are healthy as well, but people are very resistant to the idea for some reason. Emulating a natural ecosystem (as much as possible) will confer benefits to aquarium inhabitants. Anaerobic bacteria are present in natural substrates.

I'm curious why you think that plants add more oxygen to the water column than air/water contact? Maybe I'm wrong, but I was taught that oxygen is (relatively) poorly soluble in water (unlike CO2). Also, plants produce oxygen during the day and consume it at night. The point of lowest oxygen concentration is just before dawn. A tank with an aerator (or filter that agitates the water surface) and no plants will maintain oxygen levels better than one with plants and stagnant water.

It seems like you're trying to make this into an argument of "which is better overall if you had to pick one", when the point was that denitrification should receive more attention as an effective method to lower nitrates. As stated, I cannot figure out why people are so resistant to the idea.
 
Also, plants produce oxygen during the day and consume it at night.
hello; This is what I was taught decades ago in biology classes. Something like they make sugars during the day utilizing the sunlight and giving off O2 as a waste product and then during the dark phase the sugars can be used to fuel growth and other life processes.
This is why I always try to run my planted tanks with bubblers to keep the surface agitated.
 
The dark reactions can take place when there is light on with no problem. Fresh water refugiums can be run 24/7 with light.

I still stand by plants are a superior method for freshwater compared with bacterial denitrification. Even in saltwater where I think bacterial export is the best method of nutrient reduction with carbon dosing it's still not really traditional denitrification.
 
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