Very cool bubblenest!! Did they actually spawn or just build the nest?
And Roll is right, the depth isn't to induce a nest to be built, but to help the job of tending the eggs and fry to be easier (less distance up and down to keep going). But if you think about it, in the wild they usually spawn in waters much deeper than our tanks, so we're probably just babying our fish too much and they would be fine in deeper tanks. But it makes us feel better, haha.
Linda