I have 3- 2" discus in my 15 gallon and I do water changes once a week and nobody is complaining.... I also have 2 ottos, 2 flying foxes, 100-200 malaysian trumpet snails(literally), 3 yellow legged shrimp, and 2 glass/ghost shrimp in there.
They just don't generate any waste (that I can see) so for me I think once a week is fine.
The tank is planted to about 20% capacity and uses a Magnum HOT 250. And I feed Tetra-colorflakes,freeze dried blood worms (which i never see them eat), some marine food (which the LFS suggested but once again I never see them eat) and some random flakes that the pet shop suggested..which they do eat!