Lol - yes, I had marbles but it wasn't actualy for the convicts
The tank had actualy been set up to breed some barbs (hence the marbles) but I also had 2 pairs of convicts in another tank which were breeding relatively happily together but I wanted to empty the tank to put in some paradisefish fry that I needed to move out of their breeding tank immediately - so I scrapped the barb breeding tank and put in one pair of convicts. The other pair I managed to offload on a friend 
As for what they looked like - I'm not too big a fan msyelf lol - I realy don't do brightly-colored substrates - it was just the practical side of things for breeding eggs-scatterers that made me use marbles - and it just so happened that I didn't have anything else to put in as a substrate at the time when I added the convicts
I suppose it could look quite nice if the marbles are a decent color (but mine were mainly transparent but with swirls of red and green - I didn't like the combo. much lol)
As for what they looked like - I'm not too big a fan msyelf lol - I realy don't do brightly-colored substrates - it was just the practical side of things for breeding eggs-scatterers that made me use marbles - and it just so happened that I didn't have anything else to put in as a substrate at the time when I added the convicts
I suppose it could look quite nice if the marbles are a decent color (but mine were mainly transparent but with swirls of red and green - I didn't like the combo. much lol)