What are blackwater aquariums?
Blackwater is the term for a specific water type that certain species of fish live and thrive in, like wild type bettas and parosphromenus species(aka licorice gouramis).
Blackwater biotopes can be found anywhere, usually in slow moving waters or pools where acids and tannins are released from decaying vegetation such as dried leaves and branches. The tannins released causes the water to become brownish, hence the name, blackwater. This can be replicated in the aquarium using dried almond leaves, drift wood and a many other things. Think of it as adding a teabag to a cup of water.
The water chemistry for blackwater biotopes are soft and acidic.
The pH can be around 4.0-5.0. For most fishes coming from these conditions, they do not necessarily need the pH to be this low as long as they are acclimated slowly. We tend to keep our fishes around 6.0 pH and a TDS between 50-80. We use RO water and Seachem Equilibrium to bring the TDS up to the desired amount. We have also tried with tap water, using an 80-90% RO with 10-20% TAP mixed in. It may vary depending on what is in your tap water.
Although it is sometimes ideal for the fishes to keep the water parameters as close to what they are in the wild, the exact water chemistry for these fishes are not required. The most important thing is to acclimate the fish slowly and keep the water clean and stable.