Dorothee, I like this mailing of yours very much. Reading it, one can imagine how intensely you try to cover the special needs of our fishes. And there are some questions open.
First, the peat granules. Peat is not peat, the quality differs very much. Your contribution tells us that even peat granules manufactured especially for aquaristic purposes are sometimes not to be recommended. It proves that the aquarium industry is not really conscious about the needs aquarists have, but vey conscious about their own needs: to sell and to earn money. Good peat does not enlarge the conductivity, and only this quality should be used for selling to aquarists. But they sell everything what looks like “peat” and could be transformed in those granules, one package looking like the other.
Sometimes you can be lucky and get good peat much cheaper in the gardening shops (if you manage to avoid stuff which is enriched with minerals and fertilizers), but often you have the same problem. I suppose it is connected with the fact that harvesting peat is no longer as easy and cheap as it was decades ago. In principle a good development, conserving the peat swamps in our own countries. But isn’t it necessary, consequently, to think of using peat at all for our aquarium purposes? I don’t use it any longer since years. To lower the pH we have alternative means. I admit, the acids must be tested equally whether they are good for our purposes or enhance the conductivity, too. But there are forms that are quite OK, and with respect to the very soft waters we have to be influenced, only very small quantities are necessary.
Secondly, the conductivity. Your osmosis plant does work very well if you receive 9 Microsiemens/cm. I know of no natural Paro-habitat that had lower values. But for aquarium purposes I think this is too low, for the small amount of water is not capable buffering the many influences by feeding, dark-light-changes and corresponding plant activity. For most species of Paros a pH between 20 and 60 seems to be quite in order; there may be some (ornaticauda, parvulus, maybe some variants of the bintan-harveyi-group, including phoenicurus) that should be bred in water not exceeding 20 to 30 Microsiemens. So, normally you should add a small amount of tap water to the water from the osmosis-plant; but I think you do anyway.
Thirdly, water change. The first is (and you say it yourself), we cannot measure everything that happens in our tanks. Most of the daily regular or irregular procedures influenced by the feeding of our fish or the activity of plants remain hidden for us. Therefore, the regular water change that you practice every week is good, better: It is exemplary. You must be praised for it. It’s a very good example for newcomers.
But we all know that this regularity is hardly to be sustained if your array of small tanks becomes bigger and bigger. You can do so caring for three or five small Paro-tanks, but when there are twenty or more, it becomes impossible.
Now, experience shows that our fish don’t die immediately if you change to longer intervals in water change. This is indeed recommendable if your fish breed nearly continously. This is not the case in nature; they breed after the raining season has begun, breed for some weeks, and then stop. If they survive their first year (most do not) they breed again in the next season, not earlier. Mostly, the next generation is more or less the next breeding generation.
This is entirely different in the aquarium. We are used to have the same animals for years. And continous water change in combination with continous good feeding often results in continuous breeding. This in unnatural and will result in earlier illness or death. The consequence shoud be that you shoukld not try to keep every Paro-tank continously at the same optimum level, but to allow some of your fish from time to time a resting pause as they know from nature.
This amounts to the following: Beginners must learn the regular, continuous water change and the continuous feeding with changing food of high nutritious value. But having learned this, they could or should decrease their striving for a permanent 100 percent optimum of all water and food values to a system of changing levels that does resemble the natural conditions better than the all-time 100 percent system. This does not mean to forget about water change, of course not, but it may be helpful for your fish to reduce the intensity for some weeks (even months) and to enhance it again after that time in order to stimulate your fish to a new period of breeding activity.
So I conclude that you could indeed omit water change fpr some time, but you must watch your fish intensely. Illness begins hardly to perceive, Oodinium for instance could take over but you do not see it directly on your fish for long a time. If they scratch on a plant or a piece of wood it is the case. It’s better not to wait for this. Therefore, I have once argued for measuring the germ density (which unfortunately is not offered by the aquarium industry because they like dying fish: You must buy new ones). If there woud be more blackwater enthusiasts they would have discovered that market-gap, for sure. In the internet, you can find that good kits for measuring the germ density are offered by several manufacturers. They are equally easy to handle than the pH- or hardness-kits.
But you can do the most by good, close, intelligent observation.