Toirtis makes an excellent point about the bristle nose plecos africamonk. They are very safe to keep with discus. Some other pleco species will find the slime coat and shape of a discus appealing and might damage them.
As for the number of fish in your tank....
IMO what could be done and what should be done are entirely different. Since this is your first planted discus tank and you've really no hands on experience with either, less is best. Six adult discus are enough to have a peaceful pecking order and will be a spectacular front fish for your size tank.
Even the most experienced discus keeper does battle with some of the more common discus diseases. The less discus you have, the less likely you are to experience these problems. Believe me. You don't want to be medicating fish in a planted tank of that size
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I have no problem with 20 cardinals, 12 cories, 2 plecos and 6 discus in your tank. My planted 125 has the following: 20 black neons (my cycle fish), 3 clown loaches, 6 dwarf albino BN plecos, 6 dwarf cichlids, and 5 adult discus.
Stock your tank slowly. The discus should be added last and I would wait 2-3 months to do that (I waited 6 months). Since we both live in NY I'm curious where you're going to purchase your discus?
:x
