Hi,
this is Heiko Bleher and it is your (Simply discus) lucky day (because I make a few comments, and giving answers while it is sunday and we are photographing a lot of new species I collected last year... and I have a few minutes not working von my volume II... to finally come out this year...)
I noticed the Corydoras threats and must make to (very important) comments:
1. Corydoras NEVER live with discus in nature together - totally different biotopes/habitats. Thjerefore with wild discus I cannot recommend it.
2. Corydoras NEED fine substrate (except the mid-water swimming C. pygmaeus and C. hastatus and dome of the stony-living species from the barbatus-group from southeastern Brazil-Paragiay-Uruguay), that is fine white or beige sand. Corydoras evolved with their barbels which is in evolution adapeted to fine substrate (never gravel) and they always (need to) dig into such substrate.
Aything else you givem is WRONG and makes the Corydoras suffer all the time.
PLEASE remember that.
Always
Heiko