Despite being larger than many fish (guppies, small tetras), I have not had any problems with black skirt tetras becoming aggressive around them. They are very hardy fish. I purchased 3 "hi-fin" black skirt tetras 2.5 years ago when I first began the hobby, and kept them in a 5 gallon tank. Since then they have graduated to a 20 gallon community, and are still lively and healthy. An excellent addition to a calm community, they act as a centerpiece In groups of 3 or more.
Gymnocorymbus ternetzi (black tetra) Fishes Exotic
Black Tetras are superb fish for newbies, but they are also excellent fish for the best of display tanks. I have a large school of them in a 300 gallon show tank in our living room; they are heavenly and get along great with the other community fish within the tank (i.e., Glowlight Tetras, Bloodfins, Buenos Aires Tetras, Red Glass Barbs, and Flame Tetras). I've had gorgeous Discus fish and Angel fish for years, in the show tank... but the masses of Black Tetras and other fish are just as beautiful, in their own way! I've had tropical fish since my grade school years; I'm now around 60 years old... and I know top quality fish when I see them. These pert, black beauties may be common and inexpensive... but as Walt Whitman sagaciously suggested... some of the best things in life are free. These fish, like flat miniature versions of Silver Dollars, carry themselves well, looking like content butterflies. They are never aggressive when kept in larger groups. They are very hardy and stay out at the front of the tank, displaying well. I even have a picture of one for the wallpaper for my computer!
Black Skirt Tetra - AC Tropical Fish
The Black Phantom Tetra will appreciate an open environment in which it will be able to swim freely. Nevertheless, ensure that the sides and the back of the tank are densely planted. Dark-colored gravel and plant covers are also advised. Fish hobbyists should know that these fishes may appear quite dull in a poorly arranged aquarium, but a backdrop of rich foliage can bring out the beauty of these water creatures. Also, a dimly lit aquarium will help enhance and emphasize the fish’s attractive coloring.
Black Widow Tetra Fish Is Dying | My Aquarium Club