The long skinny plant on right looks like Bacopa. It is a marsh plant and regularly loses the lower leaves when grown under water.
Can't really see the other plants. provide more pics so we can see them without Bacopa covering it.
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Blackwater is a term used for tannin stained water. Tannins are chemicals released by dead plant material soaking in water. Tannins cause the water to turn yellow, brown or black, depending on the level of tannins. A cup of tea has tannins from the tea leaves. Coffee has tannins from the coffee bean.
Blackwater can be hard water, soft water or even brackish (partly salty) water.
If people are talking about South America and the blackwaters of the Amazon, they are referring to the heavily stained waters of the Amazon River that have lots of tannins in and can look almost like a cup of coffee. This water has a pH below 7.0 and low GH (generally below 100ppm).
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Most fish prefer to have some surface cover and a dark substrate and will generally show better colours in darker tanks. However, staining the water with tannins is not necessary for this or for keeping fish alive and well. Having a dark substrate (brown or black), floating plants, a picture on the back of the tank, appropriate water chemistry/ quality for the fish, and suitable companions will lead to fish showing their best colours.