They do best when kept in schools of 6 and more fish.
Most freshwater fish do not "school" as marine fish do. Rummynose tetra are about the best for this, but no FW species school in the strict sense.
So when you really evaluate your Discus water parameters and set up, paying close attention to the environment tank mates will have to endure is important. Also your wallet will appreciate it. If you find a reputable LFS, they will purchase a larger order than what you will be taking home. They will acclimate them, see who are good or bad and you'll end up with the best of the crop. I worked it out with Greenwich Aquaria. For the same price as shipping domestic or wild, I'm getting a better deal a school of heartier fish that will last longer and won't be expensive fish food.
The Neon Tetra is very similar to the Cardinal Tetra. How do you tell the difference between a Neon Tetra and a Cardinal Tetra? The difference lies in the coloration. Cardinal Tetras hold their coloration from the tip of the mouth to the end of the tail. Neon Tetras have a small break in the red lining that runs down their side. Their belly is often a white-silver coloration. Neon Tetras will not grow as large, often maximum size is less than 1″. Neon Tetras are extremely cheap and can be bought at almost any pet store or LFS. For this reason they are the best budget solution when a school of fish is required. Purchasing 50-100 in a very large aquarium will make for a stunning display.
What is the best small size schooling fish?
Top 5 Best Schooling Fish - The Planted Tank Forum
NOTE: in my opinion Rummy's are a much better schooling fish...one of the best...they are also from the same region as Discus so they can live in the warm conditions of a Discus tank. Cardinals do better under 80 degrees.The best place I have ever seen schooling fish is at Atlantis. The have a great school of sardines that I could watch for hours. It is mesmerizing. They also have tuna and jacks ready to pick a sardine or two off. Great tank, near impossible to replicate in the home IMO.Which one would be the best and what amount? Some people say that some only school at night or when there's activity in the tank. What if I added a semi-aggressive fish like a sunrise pseudochromis? whould that cause them to school? Generally it’s best if you can purchase them all at the same time. If not, purchase groups of 3 or more at a time, rather than adding one fish to the school at a time.Quite a few popular freshwater fish prefer to live in schools. , , and most should always be kept in schools. Loaches such as the Clown and Kuhli loach often will pine away if they are not kept in a school of their own kind. Even larger fish such as the prefer to live in a school.
It's a good idea to do your homework before choosing and purchasing a new fish, so know you up front if they are best kept in a school. If the fish you are considering prefers to live in a school you should plan on purchasing at least four of the same species.decide based on what you like the look of best, size of your tank and behavior you want- low, or high water presence and if you lke the look of a large school of smaller fish over a med school of larger fish.