As I've said elsewhere, some of these species are more adaptable than we give them credit for. Geo Proximus, for example are a successfully invasive species to several areas with varying water conditions. Among these is the Nova Avanhandava reservoir, River Tiete, with the following water variability:
https://www.researchgate.net/figure...-the-Tiete-River-basin-and-the_fig1_324456912
Nav is an oligo-to-mesotrophic reservoir with the upper layer of the water column well oxygenated and pH ranging from slightly acid to alkaline (6.47-8.2), conductivity relatively high (83-150 µS cm −1 ), and low concentrations of nutrients (total N 0.05-0.23 µg L −1 and total P 18.02-32.33 µg L −1 ) [29,30].
I've seen location sample studies, where similar to what you said, temperature, ph, minerals, and nutrients vary between rainy and dry season, also from year to year according to rainfall. Given natural seasonal and yearly variance, some thousands of years of human effects of varying degree, and climate and geological changes over time, It seems apparent most of these fish are not rigidly attached to very narrow conditions, although an individual population or species may be more sensitive.
Large parts of SA are not the pristine environments we might imagine, with farming, mining, dams, cultivation of non-native crops, climate change, introduced or invasive species, etc. comprising an ongoing dynamic, with some species, including discus, angelfish, some geos, some crenicichla, and others adapting to changing or new environments-- pretty much as they've always done when you read a little of the bio-geographic history of the region.
My experience is water tolerance varies with different fish. For example, due to a mistake on my part several years ago, I found that Geo tapajos red heads (or OH if you prefer) can handle pH a good bit higher than guianacara. There was a study (in Alabama?, don't have it in front of me) on aquaculture catfish that concluded they're more susceptible to experimentally introduced columnaris in hard water. From this I've seen people extrapolate a rule for all fish. It's not. For example, from what I've seen of C. gibberosa it's probably opposite (as you might expect). Also, the soft water in the study had considerable tannins, which was probably a factor. Either way, in all the variances of my fishkeeping history, I've very, very rarely seen anything that might have been columnaris, practically never.
Relevant to G. sveni, mine do well with pH in the low to mid 7s and hardness on the lower end of moderate. At least as fry, I observed if pH is much higher than mid 7s they become more skittish. When I review my fishkeeping history, my observation is that some fish that tolerate fairly high pH numbers compared to native water may grow well, breed without issues, and live without illness, but not live as long as their potential. So while I often argue for the adaptability of fish often being more than some think, my conclusion is this varies by species and keeping fish at the higher end of their pH adaptability is not always best for a long lifespan. On the other hand, when I review my fishkeeping history this varies and I've had quite long lived fish (some of them wild) kept at pH a good bit higher than their reported native water.
Imo it's most often a matter of being sensible. You can if you like, nothing wrong with it, but with most species you don't have to be a stickler for native conditions to be successful-- if you keep a clean, healthy tank and it's not at the extremes of a species' tolerance. Truth be told, people are often mistaken about native conditions, anyway, because of making false generalizations about SA waters, not allowing for seasonal and other variations, or not accounting for the natural range of a species or closely related species.
