I am quoting Robert T. Ricketts, a PhD in microbiology..
"First off, nitrate is inorganic, not organic, so they start out with an error, a.k. a. a lie.
Then they go into the biggest bunch of BS I have ever seen in my life. If your tap water is hard and alkaline you cannot use it for a planted tank? I have never had soft acid water in my very long hobby life. I started out in the 1940s and early 50s with planted tanks. They say you would have to buy DI water to use. Another lie and terrible advice. DI water is unsuitable to sustain life, either plant of animal without supplements, and it does not have a pH of 7.0, it is always acid from dissolved CO2 gas from the atmosphere."
"All existing denitration filters (other than veggie filters or algae beds) require low oxgen levels.
BUT, (note large type -intentional) just what have gained by removing nitrate from your water without doing so by means of photosynthesis? You have removed the one easy and simple test which we have for general water quality. That means that you are hiding the truth about your water condition from yourself. Using chemical or biological means to remove nitrate does not necessarily mean that you are removing the myriad other poollutants from your water. Even heavily planted tanks need water partials. Photosynthetic nitrate removal does also remove a lot of other material from the water, but certainly not all that a water partial does. I really prefer not to kid myself. I know what a water change does and I can measure it. I do not not know what a denitrification filter does beyond removing nitrate alone.
All JMHO, YMMV.
But never forget that nitrate alone is not the enemy. It is the only thing that hobby test kits can measure easily from the dozens to hundreds to thousands of things which we call pollutants and which we can measure easily and cheaply."
"Either a DSB or a plenum will denitrify with reduced ORP (oxidation/reduction potential), but the plenum is safer if there is any chance of disturbance - such as FW. SW w/LR is safer for a DSB due to massive assortment of diggers which help alleviate deep/profound anoxia (sulfur and iron reduction are profound anoxia/very low ORP and chance disturbance can be a tank wipe-out).
But too many hobbyists equate low NO3 levels with no need or reduced need for water changes, which is a grave mistake. I had rater see the nitrate and know how well I am doing on routine partials.
The reason I came onto the hobby web originally was to search for info on plenums and DSB in other than marine conditions (the original Monaco work). Prior to that I had only been on the gov/military system. I di a good bit of hobby work on plenums and found that they do work fine, but too many folks have troubles with them and they are painfully slow to establish. Some (not my own) never did seem to function. I am unsure why not but suspect owner or fish disturbance - folks tried the technique and then added Kuhli loaches or eels or big substrate Cichlids....ten percent never get the message.
Such issues are also why I prefer to use sumps/circulating range systems. There you can cut the test equipment in or out as you wish - which is a great test platform. it is not hard to show that nitrate alone is of low short-term toxicity and only moderate long-term toxicity. It ain't the nitrate, it is the whole OTS water issue which is bad. The degree of toxicity also varies hugely with the specific fish involved and their age, and even with how rapidly the situation develops. Too many variables are involved for hobby testing to be very meaningful over a wide range of tanks.
So after all those test tanks and several years of play, I went back to veggie/algae filters - I set my first SW algae filter in the 1960s, connected to my first marine invert tank. That is now on the order of 50 years of playing with denitrification. I never did like the vodka coils - too fiddly and relatively high risk.
Veggie filters at least allow playing with the light cycle on the accessory tanks and supporting higher O2 levels in the operating system as a whole. Again that is less significant in SW due to the presence of skimmers, but 24/7 macroalgae refugia are still great additions - especially if can set for passive return to the display (preserve the 'pods)."
Regarding "trace elements":
Yes there are trace elements that need to be replenished in a system and you are correct in the reasoning as to why they are called "trace", but calcium, magnesium, sulphates, and phosphates all play a roll and are NOT trace elements and are consumed in the process as the tank matures. Regular water changes in a FW situation helps build these elements back up. Some may be more or less desirable depending on the tank you are keeping and may be supplied independently of each other.
So ... in a typical tank ... where does the fish waste go? The TDS would build up over time and removal of it is necessary via a gravel vac and water changes. Playing with the number until you achieve a desirable level is akin to spraying a room with deodorant after the dog has pooped in it. The stuff is still there.