Pond rocks
wash em and scrub em like taters, then boil them and let them sun dry. Then do the bucket of water ph thing...
Taking out that gravel, remember that there is bacteria starting to grow on there that is beneficial to your tank. Cant really speak much to sand as I always stayed with a thin layer or polished rocks. Easier to vaccum
One thing to remember with rocks, that I learned with my piranas. When they grow a lil bigger (14"&17"), they're going to move things around to how they like them. Make sure they're set pretty sturdy, or you'll start hearing the rocks slamming into the glass at odd hours...
If you want to see that convict grow quick feed him a weekly cycle of krill in a month