Referring to the subject, it is best that discus should have the whole tank to their own but that doesn’t mean your fish do not deserve other tank companion. By nature, they are very shy and slow moving. As such they do not prefer to have other active species swimming in the tank. Nevertheless, you should also watch out which fish you decide to mix with them. Certain tropical cichlids like oscars can wreck havoc or maybe attack your discus fish and therefore, you should not mix both of them together. Based on my own experience, discus is best raised in a tank of their own. There are few exceptions however.
Discus fish are known to leave leftovers foods littered around the tank. Compared to other fish, although they have relatively small stomachs with short digestive system their feeding habit always leave something around. Food that you put into the tank are usually live worms and fish meal that doesn’t stay fresh long enough in the waters to be consumed. Some of it will be eaten while the rest will just stay there and goes untouched. As such it is best that there should be a companion that takes care of the cleaning job to ensure nothing gets left behind. For that, pleco, which is a type of catfish family, does a good job in this area. However, in a typical discus tank setup, there should only be one single pleco to accompany your stock or a group of discus fish.
Clown Loach is another possible good tank mate for your pets. Since they are generally bottom feeders, they activity around the tank will usually go unnoticed by your discus fish. Similar like pleco, clown loach is also good at picking up the small pieces of uneaten food and thus should be good companion. However, one thing to take note is that try not to outnumber the discus with a group of clown loaches. This could lead to heavy competition and fighting during feeding time.
If you have a heavily planted aquarium setup, you can also consider adding neon tetra even though their numbers tend to dwindle over time due to possible harassment by your discus. Although the smaller sized neon tetra will hide itself trying to avoid your discus, sometimes it is possible that they will end up as food instead. Therefore, it is best that not only plants are present but you should also prepare some artificial caves for your neon tetra to hide in. And because both species belong to the Amazon River, requirement in terms of water quality is more or less the same with both preferring slight acidic water. One benefit for sure if you managed to get your discus to coexist with your small neon fish is that, this will definitely create an exciting community aquarium that will be interesting to look at.