Hello, so i have a 20g long planted tank with 2 bolivian rams, a dwarf gorami, 2 killifish, and 6 black neon tetras. The black neons are starting to get old and I am thinking of replacing them with 6 congo tetras. What is your opinion on this? Any advice is welcome.
I've seen plenty of male Congos over 4", I would not put six in a 20 long. However, you might get 5-6 Diamond Tetras to work in there. They are deeper bodied, and have essentially the same colors as a Congo, but with some spangling. Males also have longer fins like the congos. They are also not as frenetic as Congos can be.
They are probably the tightest schoolers and with enough open space they will swim back and forth in your aquarium all day long. The only drawback is that they are really difficult to acclimate, so you need to make sure you get them from a reputable LFS and that your water is pristine.