Dempsey nest?

[Jinxster373]

I have a male and a female Dempsey. They Are still young and no where near full grown size. Have had for about 3 years. I use a gravel and coral substrate which has been working since day one. I also have marbles and coated sea glass that is not sharp to the touch. They pick these marbles and glass up and constantly make I guess a nest? But I have never had eggs or fry. Do you think that a nest is their intention? When I do water changes I move the marbles and evenly distribute them
And they always end back up in that same exact spot!! You can see in the one picture how there are like no marbles because they have moved them all to the other concentrated location. What the heck are they doing!!??