agreed.
Tap water contains things the only last for hours at most with circulation like chlorine. Plant food ammonia, nitrates, phosphates. And parts per billion of some nasties. All things our systems will remove from the water column with a thriving plant growth. Just as the enviromental engineers remove toxic wastes from polution with living plants (FW) and dead saltwater macros. And use various marine algaes including various caulerpas as bio indic
ators. To monitor copper containmination upstream of copper mines and power plants.
So by starting a tank with thriving plant life the water column rapidily becomes suitable for all forms of aquatic life. But it does take a couple of days to a few weeks for things like copper.
One thing I am absolutely certain of is that all the stuff in you input water does not remain in the system for ever and ever killing all life.
And I even agree that processed water does have its place. But that place is not the establishment of new systems or to simply prevent plant life like the various micro algaes. The way to handle both of those is to establish plant life you like.
But I have seen numerous arguments on these boards where someone used to use tap everything was fine then they moved to another location and the tap was terrible there. Dah!!!!!!!!! The problem was not the new tap water but the fact that at the new location the tap had not yet been processed. So a previous thriving system had 80% of the water changed with unacceptable water. What should have been done was to either use ro/di as the new water and then return to the tap for top offs and the small water changes. Or to slowly refill the system at the new location to the system could process the tap as it was being added.
At most RO/DI is only necessary when you change a very large amount of the water in the system. At that sould be extremely rare and extraordinary circumstances.
Well possibly when you are raising some delicate FW fish that require the very soft water in pristine streams and lakes to say induce spawning. But that is not our salt/reef tanks.
I think it is possible that tanks get old. But it is not the water. And macros do regenerate frequenty. So you harvest them once or month or so. I have also heard of old tank syndrom being a DSB consideration. Where things just build up over time in the sand then the system crashes. One planted marine tank was in operation for something like 8 years but they did a 100% water change per year.
So old tanks can be a problem. But once the system is balanced initially with plants IME they just run for years and years. Unless I play with then, tear then down and move to another city or whatever. All I do is just add food, provide light and keep the system topped off.