Hi Tom, interesting how you so often seem to have the definitive last word on these threads! And certainly helpful to have an expert opinion, so thanks.
In any case, following extensive discussion by many here as well as on the Why do water changes?
thread, I'm having to conclude that water changes are not necessary for all tanks, (dependent upon plant growth, filtration, bio-loading, resultant water quality, etc). They may certainly be beneficial to most systems which are not balanced and which therefore experiences build-ups of one type or another, but of course many people have success without water changes as well in properly planted and stocked tanks.
And despite all the hypothesises to the contrary offered here, nobody seems to have any firm idea (or evidence for that matter), on why water might become 'broken' and have to be replaced in time. I'm just going to have to go way way out on a limb here, and suggest that unless water has its molecular bonds between H and O atoms wearing out in time in our tanks for some peculiar reason, then it can continue to be re-used indefinitely.
Of course proper filtration/planting, and treatment including some remineralization, buffering, etc would continue to be necessary. But those treatments and remineralization would be even more necessary for the RO water I'd have to use for water changes here anyway.
I was at first certainly of the mindset to use CO2 to produce amplified plant growth to match the amplified bioloading. But then it had also been suggested that certain floating plants utilize atmospheric CO2. Therefore, I could instead harvest fast growing floating plants to cycle nutrients out of the tank, without even requiring the use of C02 to achieve intensive growth from them, (I believe). I already have a digital PH controlled CO2 system now, so I wouldn't mind using it at first to get the plants to a quick start and then using just a little bit as seems appropriate to keep the decorative plants looking good but without excessive growth.
I'd be truly delighted if the plants did all the work and I didn't need any complex equipment, but so far as I'm aware I believe that low bio-loads and low feeding are required for that to work. So I'd like to have the safety net of a bio-filtration system with an extensive capacity as well to appropriately sustain a decent amount of bio-loading. And especially so if I'm to pursue aggressive growth rates for discus with numerous generous feedings daily. As they'd reach adult size then the filtration might be able to be pared back though as seems appropriate, in case the filtration was competing excessively with the plants for nutrients.
I don't have any aversion to dosing itself, but I was concerned that if I'm not monitoring the levels of each trace element then some of those could build up excessively in time if I wasn't careful. And it seems that for that reason others have suggested mineralized topsoil under the sand, which it seems takes the concern of dosing build-ups entirely out of the pictures. MTS apparently ensures the plants have everything they need without those elements being in the water column, which apparently helps to minimize algae growth. If I didn't go with the MTS then I would be dosing myself though, which would be fine.
And to be honest, I would probably be automating the dosing of food to some extent. With discus I'd want to still do at least one or two feedings of frozen food a day, but with an automatic feeder I could add an additional two feedings a day to give them a better chance of obtaining their proper size.
I'll have to think about it carefully, but I suppose that if I set up a 29 gallon tank in the stand then I could use that to automate whatever water changes were necessary, as well as use it as the quarantine tank in case I didn't require its constant use for frequent water changes.