In a tank that size I would go with a trio of apistos, a school of nannostomus eques or tetras (if you have no real desire to raise fry) and some corys. Just my suggestion because I am an apisto guy. Filtration through peat will also help you achieve your desired ph(I've never gone that low for a display tank, just when I was breeding a difficult species otherwise I kept the ph around 6.5). Frogbit or some other floating plant are nice additions to blackwater aquariums.
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