Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Those are all soft water (low GH) fish. Many Cories do not care much what the GH is, but they come from softer water, and the more delicate ones (Pandas, for example) seem to need it soft. Otos are mostly still wild caught from very soft waters, and Discus can go either way. Wild caught or recently bred from wild caught need soft water. Discus bred in captivity for several generations do fine with harder water. I am not sure about the shrimp, many are soft water, but research the ones you want.
If you want to do nothing, that is OK. Go with the hardier Cories (Brochis splendens will tolerate the warmest water, and handle harder water), perhaps Bristlenose Pleco instead of Otos, and captive bred Discus.
If you want perfect water (if there is any such stuff) I would mix your water with RO or Distilled about 50/50 to drop the GH to 3-4 degrees.
Then add enough potassium bicarbonate to bring the KH back up to 2-3 degrees.
Then filter the water through peat moss to add the organic acids these fish like.
This ought to bring the pH down into the mid to upper 6s. Not an absolute requirement for the fish. GH is more important than pH.
This way you can keep the more demanding Cories (research temperature ranges), Otos (Do they handle the high temps Discus need?) and wild caught Discus, or Discus bred in soft water. Probably a wider list of shrimp.