I was thinking about adding geophagus with discus in a 180 gallon tank, but after seeing how my 8 geophagus ate so robustly and quickly, I would never consider that again. Discus are timid, slow eaters, there is no way they can compete with geophagus for food. I would like to see a group of discus that has been in a tank with a group of geophagus after a year... my guess would be that they are stunted from not getting adequate nutrition.
Also, while I have seen people often recommend clown loaches with discus, they did not work with my discus. I tried. The clown loaches cornered and chased my discus, harassed them. Not sure if they are attracted to the thick mucus that the discus exude or just bullying, but, regardless, I will certainly not be doing that again.
These are the fish that I have successfully kept with discus, the list is short because of the nature of discus: their slow eating and passive nature, the high temperatures required, and the need to find fish not attracted to their mucus layer.
*Rummy-nose tetras ( a school of 100) ( excellent choice if want tight schooler.).
or
*Cardinal tetra ( a school of 100)
Have you ever seen a sight of this many small tetras schooling together- like birds in the sky- it is beautiful.
This is the only two tetras I would recommend. The bigger tetras like Congo's and Denisoni will eat too vigorously at top of water. You will be constantly trying to make sure your discus get enough food. Discus are hard enough, why add fish that make feeding more difficult.
Most corydoras species cannot take the higher temperatures, but these can: Corydoras duplicareous, adolpho, davidsandi, sterbei, agassizi, and weitzmani.
Plecos:
Ancistrus sp., Hemiancistrus sp., and (some) L. numbered plecos that can take high tempetratures. But, all Plecos need to be monitored to ensure not attracted to discus mucus.
I have heard that angels go well with discus, especially Altums. That could also be a fish that would be worth trying as many have had good results.
Thats it ---like I said, the list is short.