Something has come to my attention that some fish keepers tried to keep multiple gourami in the same tank. It is a big mistake!
This is from my first hand experience, that Dwarf gourami (or any other gourami for that matter) should be kept away from each other. When you have more than one gourami in the same tank, they will be aggressive toward each other because they are a territorial fish species.
The infamous Siamese Fighter Fish (betta) is in fact in the Gourami family. Many people might know betta should be kept alone, they really don't know about gourami are the same case.
I did not know better when I first started my fish keeping hobby, and I foolishly got 6 dwarf gourami (3 male and 3 female) for a 40 gallon tank. It was a free for all fighting. Male attack male, female attack female, male attack female... In the end, I had to return 4 of them and left only a pair of 1 male and 1 female. Although they spawned together, they weren't exactly on friendly terms even during the spawning. I had to separate them as soon as they stopped spawning (as I should).
The offspring weren't peace loving siblings either. Those home bred little dwarf gourami started to show aggression toward each other as early as only 4 weeks old. At the time, they did not even look like fish yet.
At 8-week-old, one male dwarf gourami just started to show color, and they've become more aggressive than ever toward each other.
Check out this video. Look at these two dwarf gourami siblings.
Although in this video you see the aggression is one sided from the brother (the bigger one with red color), off the screen I have caught the little sister attacking her brother on at least two occasions. She was sneaking up from behind and tried to hit him.
The conclusion for all fish keepers, please do not put two gourami in the same tank in order to avoid unnecessary injuries or even death of your fish.