Acceptable Nitrate Levels.

[mcordelia]

soooo, you're recommending a walstad tank approach, which is a well documented and working tank model, however not frequently utilized because most people prefer having more fish than what is considered sustainable in a walstad tank. However, perfectly valid approach, and everything is on a spectrum, so one does not have to ascribe to a "true" walstad tank without reaping benefits of the methodology

I'd be typing, and quoting various links, for days to cover it all.

go for it. We love discourse here, and believe that advancements in the hobby arise from a comprehensive understanding of the science. The scientific method itself is founded in debate, since no theory can gain eminence without attempts to first prove it false.

It is 3 molecules.

please. If you pull the "fifth grade science" line, don't make mistakes like this. water is composed of three ATOMS, which make up one water MOLECULE. And honestly, tank water is made up of a helluva lot more than three molecules. you have your H2O, but you also have your dissolved O2, and you also have your carbonic acid equlibrium (http://ion.chem.usu.edu/~sbialkow/Classes/3650/Carbonate/Carbonic Acid.html) (which, by the way, is a vital equilibrium to maintain humans alive since that is how your body deals with CO2 generated by your tissues), not to mention trace minerals which affect your hardness and nutrient contents of the water.

Not doing water changes is the holy grail of aquarium keeping, and I'm impressed that you've been able to maintain tanks for years with your approach. However, in order to find the correct elements to get the equilibrium working without a crash, proper understanding of water chemistry needs to be achieved. On one hand, it's a good start to tell new hobbyists to "get a bunch of plants" and hope for the best, but because water parameters etc. are so variable, it is unlikely that such an elementary approach works for everyone across the board, which, then again is ultimately bad fishkeeping because anecdotally this experience works for you, but can not easily be replicated for everyone which is where the crux of the problem lies. I suspect your water may be relatively soft, which gives a large reserve of "space" to allow for mineral/etc buildup. This type of water also has a propensity to turn relatively acidic however, so I would be interested in what your parameters are in your tank water, to learn more about your method.

Have you read Diana Walstad's book?

 

[AbbeysDad]

Don't overthink. Just look at what nature does, and do your best to copy that.

I'll grant you that as written above by @mcordelia, a pseudo Walstad method can work, although following the publication of her book, I understand that Dianna made some modifications to her approach.
But the average aquarium is typically not heavily planted with fast growing plants and only containing only one or two small fish.
I think if inexperienced hobbyists were to try the Low/No Maintenance Aquarium, it would most likely not end well.
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Topping off a tank to compensate for evaporation is not anything close to what nature typically does. In most cases, fresh water is constantly replaced as rains and snow melt feeds streams and rivers that flow endlessly directly or through lakes on the way to the oceans.
In nature, the planet recycles water and fresh water is made fresh by this process.
So if we were to really look at what nature does, we would have flow through systems or do partial water changes much more than once a week and/or in much larger volumes.
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There are some ponds, swamps, and ditches where water evaporates, only to be topped up if/when it rains. But many of these are often pretty rank, and many dry up completely until the next rainy season.
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So you may have found a recipe for a low/no maintenance aquarium but I have to say that your view of nature relative to fresh water is a bit off.
I too would be curious as to the pH, gH/kH, and phosphorus levels in your tank water(s)...as beyond nitrates, these tell us much about the water quality. (sadly without expensive equipment, we don't have ways to check hormones, pheromones, and heavy metals.)