Thanks to all for the input!
Part of my problem is that I (mistakenly) tried to adjust water to suit incoming fish, when I should have let them adapt to my conditions.
But in addition, I would like to try breeding my Satanopercas, and I’m not sure how much I would need to change for them. Is it reasonable to rely on the TDS meter, mixing RO water and tap water to make fairly soft water (80-90 ppm) ?
Ya know, reading this post ^ makes me think that the modern age of fishkeeping, in which many people are ordering fish by mail from all over the continent and maybe beyond, is exacerbating the whole right-fish-wrong-water issue in a way that didn't exist back when all or most fish were purchased locally.
I still buy everything locally, either from a local retailer/importer about an hour away from my home or from local breeders who sell/trade at club auctions. So every fish I buy...and I don't buy that many!...was either born and raised in my local water (which is not hugely different than the water from my private well) or at least was brought in by an expert with many years of experience acclimating many hundreds of thousands of fish from all over the world. Either way, the individual fish that end up in my home tanks come home in bags full of essentially the same water in which I plan to keep them. That's a big advantage, IMHO. When that local importer gets a shipment of 500 wild-caught Cardinal Tetras (just a random example...) that were caught a few days or weeks earlier in a river with a pH of 6.2 and maybe 30ppm hardness...and were shipped in that same water up to the Great White North...they were then forced to acclimate to a pH of around 7.5 and a hardness that easily tops 300ppm. How many die in the first few days after arrival, even under the expert care of a quality importer?
Then a few days later, I or some other poor schmuck takes a bag of them home. It's only an hour or so drive, and the water in my tank isn't hugely different than that in the bag...but the fish are still reeling from a long period of hardship and stress. Maybe a few more die...and maybe some more will show long-term deleterious effects in upcoming years (in larger long-lived species, like Oscars) but the worst part of their ordeal has already passed by the time I get them into my house.
Contrast that to a typical buyer today, somewhere in the U.S., who orders fish by mail from all over the country. Surely there are some places in the country with soft/acidic water, similar to the water the fish originated in, so the fish suffer less of a shock when they first arrive in the country, and that's what those fish are packed in for the journey. Then they get to your house, and your water is likely closer to mine in chemistry...so that single biggest acclimation hurdle is faced by the fish when you open the bag at home, rather than when they first arrived in the country. Those initial deaths due to a sudden change in water chemistry are now going to occur in your living room rather than at the importer/distributor, aren't they?
I'm just thinking out loud here; no idea if this idea has much or any validity. It seems worthy of consideration to me. Would love to hear some comments from others with a better grasp of the logistics of importation.
Sorry to derail your thread,
@Gershom , but it seemed at least semi-related to your original question.
Edited to add: I wonder if it's a coincidence that African rift-lake cichlids make up a disproportionately huge percentage of the fish my local guy stocks...or if that's related to the fact that our water is pretty good for those fish, straight out of the tap?