To create a blackwater tank you need to add botanicals. This will alter your PH to the very acidic side all the way down to 4.0 range is possible (not a bad thing) and your bio filtration will rely more heavily on fungi than bacteria at this point for nitrification. The tannin release from botanicals such as leaves, stems, wood and seed pods will turn your tank a dark tea color making it "black water". A blackwater environment is ideal for most tetra species and many south american fish species in fact. You simply allow the botanicals to break down slowly in the system. Twig catfish and plecos will also munch on them.
You can find lots of good stuff and more info at Tannin Aquatics online.
Welcome.
