It's been interesting, and eye-opening, to see everyone's POV here. But I don't think we're actually getting anywhere except that - we all have our own thoughts on the matter, and won't be swayed.
One last comment, and then I'm going to unsubscribe from this thread, as I don't think there's anything left for me to learn here. . .
Granted that all water is recycled from the beginning of time. Dinosaur pee, people poo - it's all inside, lol. But most of my fish are wild-caught, and come from areas where the water is far cleaner than anything I can ever hope to offer them in a tank - well planted though it may be. I brought these fish here, and I consider it my obligation to them do my very best to keep their environment as pristine as I possibly can.
Because it is so important to me to keep them in the cleanest water possible, I've done a lot of reading and research into water - as it seems we all have - and gone as far as to take a tours of our local water treatment facilities. While it isn't perfection, I left those buildings with a greater peace of mind than when I went in, and with many of the questions I had answered. Not answered with a route 'this is what we're supposed to tell you' answer, but in many cases, with replies that I had to come home and look into, as the science was a bit beyond my personal understanding. I was happy to come to the conclusion that these people knew what they're talking about, and our tap water is as pure as they are able to make it with the technology available to them.
My tap water comes from three different reservoirs/dams, and to my tap with no detectable traces of ammonia, nitrite, nitrate, or phosphates, except that during certain times of the year when the rain is particularly heavy, or in the spring when every one is planting and spreading fertilizer all over their yards, there will be an occasional low nitrate level.
I've tested the rainwater several times in my area, along with the water in all of the nearby natural streams and waterways that were close enough for me to entertain the idea of hauling buckets from every week (I considered using 'fresh' water and not tap for a time for water changes. Gave up on that idea). Here, the rainwater brings traces of ammonia with it - and the 'natural' water is harder, and depending on how much rain we've gotten, can be . . . not so fresh (oddly enough) as the processed tap water . I guess that's what I get for living in a big city, but I'll take it! Point being that around HERE,
anyway, the dinosaur piss in the rainwater is somewhat obvious - though not so much in the processed tap water!
That said, I'm not a very scientific-minded person. I try, but I'm an artist and a writer, and my brain goes all funny when I try to process some of the papers and research that I've read. So I have to rely on what I've seen and experienced in my own tanks to really gain the understanding that I seek. And what I can tell you, first hand, is that on the few occasions where I've been late for a water change, or missed a week entirely, the behavior of my fish is different. Nothing obvious like gasping at the surface, or huddling in the corner. . . and I've never let it go long enough for there to be any differences in my API tests, either - the levels read the same - even for nitrates and phosphates. But the rams start start to lose their color - just a tad, and those lower on the chain begin to show slight traces of their 'stress stripes', and the loaches tend to be less active. . . the tetra seem . . . restless, and not as vibrant as they generally are. It's all very slight, and really nothing that most people would ever pick up on. My husband thinks I'm crazy, because he can't see it - but it's noticeable to me, because I watch my tanks so much. It's slight, but they all just seem not *quite* as happy as what I'm used to seeing from them.
From reading this thread, I guess NOT doing water changes works for some people - at least to some extent, and for some time. And as long as your fish aren't obviously suffering. . . that's your way. Hats off to you. Maybe it all comes down to what the fish are used to - I don't know! But based on what I've seen with my own eyes, I'll stick to my 30-50% weekly water changes, as my lil' buddies seem happy with things the way they are - and when they're happy, I'm happy.