I am so tempted to tell you to provide them with some bamboo thickets but I won't :lol:
A friend found the most successful way to breed corys was to put them in a small tank, let the water go very green, and then forget they are there. When she went back, 2 or 3 months later, to empty the tank there were a lot of fry all happily living in there. The fish had never been fed but obviously had enough algae to live on