Imo the study helps explain some things. My thoughts extrapolated from the article:
1. May help explain why people keeping blackwater fish in low pH tanks find they need to also keep nitrates low for fish to stay healthy. I'm NOT saying this is the only factor or that it takes other sensible things out of the picture, such as the need for very clean water, low bacteria load, and the apparent benefits of tannins.
2. Might (help) explain why it seems you can get away with higher nitrates with something like African cichlids, typically kept in hard water, higher pH tanks. I used to know (through a forum) a guy who for several years was highly regarded as a Cyphotilapia breeder in Australia. Due to water restrictions he waited for nitrates to hit 50 before doing his water changes, yet his fish were quite healthy and his tanks were immaculate.
So I think a few things here. First of all ignore whoever tries to tell you high nitrates do not affect your fish, don't like to say this but it's hard for me not to consider them idiots-- or maybe contrarians out to prove a point, which occasionally amounts to the same thing. Second, what's "high" for nitrates depends on a few things. You can't blissfully keep Malawi cichlids in pH 8 and nitrates over 40 (which might be okay when everything else is in order), think you've got this aquarist thing whipped, and try doing the same with S. Daemon. On the other hand, if you find your soft water, pH 5.5 Satanoperca tank has issues when nitrates go over 10, don't make 10 the cosmic limit for everyone else's tank.
As for the pH thing, I find it varies,
but-- based on the loads of empirical evidence, for many species it's clearly not necessary to replicate what we
believe to be their native pH or hardness. I emphasize
believe to be because I see a lot of mistaken assumptions out there (e.g. discus all come from very soft. very warm, low pH water-- some do, others don't), and because the main river we may associate to a species isn't always the smaller tributary from which it's collected, where pH and hardness may be quite different. And species simply vary in adaptability and the range they can comfortably handle (why can't people get this through their heads). I have some wild A. metae from Rio Atabapo, typically black water fish. I've moved them around from tank to tank a good bit and they could care less about pH or hardness (up to 8, anyway). I had wild Heros sp. rotkeil, same thing. Same with the Geo pyrocephalus I bred for at least a dozen years. On the other hand I had some Guianacara that could not handle high pH. Some are more adaptable than others and ime it clearly varies by species...
I've mentioned this Amazonas magazine article on breeding Geo mirabilis before, read it back when, can only find a partial reproduction lately, but here's his parameters: TDS 480, GH 340, KH 100 (as close as I can make out), nitrates "under 40 ppm," pH 8.3. Not what I'd do but there it is 'in black and white.'
https://www.amazonasmagazine.com/2017/07/13/amazonas-video-bonus-breeding-geophagus-mirabilis/
duanes
-- I like to do large water changes and I agree with a lot of what you're saying, but assuming good filtration (also feeding sensibly, not overstocking, etc.) my thing is there's not a good comparison for water changes needed vs. river flow, etc. Which is fortunate, otherwise we'd
all be in trouble. I have my own sweet spot for my tanks, often do more than that, but with SO many variables I don't like to get too specific making recommendations.