I'm very interested in what you say. It's not that I'm interested in duplicating your setup; It's more of a curiosity from a tank chemistry point of view.
My main question is this: You feed your fish with food I presume. And with that food you are bringing nitrogen into your tank in the form of proteins and whatnot. Unless that nitrogen leaves as a gas (such as nitrous oxide), it will always be in your tank, no? How do you stabilize total nitrogen?
Hopefully not nitrous oxide but n2.
and thanks for your interest.
Yes I add food and therefore nitrogen along with phosphates and a host of trace elements also.
What happens is the aerobic bacteria ammonia->nitrIte->nitrates cycle is completed with nitrates forming plant tissue in the macro algaes.
At that point nothing more needs to be done as all the food has been turned into fish/coral/ and plant tissue. In fact the plant tissue is eaten by some of the fish therefore the nitrates are recycled.
After some time the macros do get big enough I remove them. Which remove not only nitrates but also trace elements and heavy metals like copper the plants have bio accumulated. Some you will have to replace (calcium) and others you want out of the system anyway.
I suspect that if it were not for some tap water additives with things like zinc, you could even do sps corals by balancing out the tank with macro algaes. But that is pure specualtion on my part.
I have no doubt tap water from any city in the US can be used for marine and FW fish tanks with plant life and no water changes. I have done it already for years at several cities in the US since 1970.