A blackwater tank uses tannins from driftwood, leaf litter and seed pods to create soft, acidic, tea-stained water that mimics the natural habitat of many tetras, dwarf cichlids and bettas.
The tint comes from natural botanicals releasing tannins, which gently lower pH and are thought to have mild antibacterial benefits. It is an aesthetic and a biotope choice, not a requirement for most fish.
It suits soft-water species; it is not something to force on livebearers or other hard-water fish.