I've come to challenge a lot of these 'old school rules' of fishkeeping
(not to mention a lot of the myths that are repeated over and over:
> 1" per gallon
> 4x-10x filter flow rate
> over filtration
> more filtration = more fish
> ......).
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I think a pair or trio of Swordtails can live just fine in a 20g tank (and I'm likely the only one here growing out 100-200 Swordtails!)
Now my water is neither hard, nor soft really with a pH of 7.6, but I've had Angels, Neon Tetras, Cories, Swordtails, and Mollies all in the same planted tank living just fine.
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Many/most of the fish we have in the hobby have been bred and raised in a wide range of water parameters. Will some fish do better in soft, acidic water and others do better in hard, alkaline water? Perhaps, especially any wild caught fish, but I feel that sometimes we can get too caught up in a rigid mindset that just doesn't make sense. I have bred and grown out Angels to adults in my water...does it really make sense then to think these fish can only live in soft, acidic water?
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Water just way to soft or liquid rock - so you fix it with Equalibrium or cutting with RO or distilled water with each water change - where there's a will, there's a way.
I have high nitrates in my well water....so I pre-filter through API Nitra-Zorb to remove it.
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As to water quality it comes down to bio-load and water change frequency/volume. A well maintained 10g tank can potentially have better water quality than an average 100g tank.
I shake my head when I see posts of hobbyists that have larger (medium to heavily stocked) tanks with canister filters that they only service every 2-3 months or so and only do limited partial water changes once or twice a month!!! Water may look 'clear', when in fact it's very polluted!
