Word of caution with large river rocks - - a poorly timed kick can send a rock slamming into a wall...forceful enough to even crack your aquarium glass
Well that's no good *sigh*. I don't want to hurt them but I really don't want bare bottom. Do you have a personal preferance as far as substrate? When you use sand do you witness them eating large amounts?
I read about pool filter sand being used for fish because it's less able to damage filters plus requires less rinsing than playsand. Do you have any opinions on that type of sand?
How big across would rocks have to be so that an adult can't eat them?

Thank you for the sexing info, now the questions will be whether or not to hold off on naming until they're old enough to sex or to find some names that could apply to either sex.