Some cracking looking marine tanks here, something I've always wanted to keep, but out of my price range, but one of these days....
I'm now down to just the one tank, 6x2x2 with a male ornate bichir the sole occupant, been a nightmare over the years trying to get some sort of permanent tank mate for him, tried various cichlids from blackbelts, loiselleis, argentea and Cichla pinima, he tolerates them for so long then decides it's time to try kill them

Even tried an RTGG, had to move it on, at 12" couldn't cope with ornate. Would love to turn the tank into a big community tank but couldn't part with the ornate.
Not the greatest of pics, but this is him
I notice some use gravel and some use sand, I use sand myself and couldn't see myself going back to gravel, it takes 2 mins to give sand a quick stir about with an old net frame to prevent compacting and gas buildup.
Bandyquill have you tried selling your gibby on aquarist classifieds? I've moved some fish through there in the past, and there's usually something that'll catch your eye too.