I hope I always have Serpae Tetras in one of my tanks. They are common and inexpensive, they look beautiful and natural at the same time. They are schoolers, so buy them in groups.
Sometimes a Serpae will become a little nippy, but it is usually harmless. The females get a little bigger and rounder, and the males stay sleeker.
They don’t get big like Bleeding Heart Tetras so they are good for smaller tanks.
Many years ago I saw a tank at Shedd Aquarium in Chicago that had at least 50, and maybe a hundred, Serpae Tetras. I doubt that you can look underwater in the Amazon River and see 4 species of Corydoras, 4 or 5 species of Tetras, and another 5 species of fish all at one time. More than likely you would see a large school of one or two species, like that tank at Shedd.
I always pronounce their name like Sir-pay, but I have heard them called Sir-peas, and Sir-pies. I’m stickin’ with Sir-pay Tetras.