I have noticed in the past few weeks that both my Oscars ( an albino and a tiger) have been scooping up gravel with their mouths near a cave I have set up for them in my tank. They have done it so much that they have dug through about 2 inches of gravel. Is there any reason why they would do this? Is it some kind of mating habit? I'm not sure on the sex of my oscars or if they will even mate due to the fact that they are different breeds.