Is it okay to put colored
marbles in a bowl/jar with a betta?
pros and cons of putting "stuff" in your betta jars :)...
Whether you chose to add no gravel, fine gravel, coarse gravel,
marbles, or any other cheesy ornaments you can find at your LPS, I
really don't think your betta cares either way. In truth, the
ornaments are probably more for the betta owner own sense of
esthetics than for the betta itself :). So the question really is
"will it hurt"? Here are a few things to consider before
taking your decision.
Now my two cents: Personally, I would favor very fine gravel (as in river bed) of natural color. It can be hard to find but a good fish store should carry it. With fine gravel, the jar will look pretty but you will still be able to see uneaten food because it will lay on top of the gravel bed (as opposed to inside the gravel bed). Live brine shrimp can't hide it it. Worms may dig themselves in, but with effort and Mr.Betta is likely to get them first (hehehehehe crunch crunch yum yum). Also I would add a nice live plant, which will make your jar look pretty put will also help keep down pollution (So is useful to the fish and ecosystem). Just make sure to keep your plant healthy, cut any leaves that are dying, and replace it promptly if it deteriorates too much. Always favor plants with medium leaves as opposed to "needle" type leaves. These can be a real cleaning nightmare and shed like crazy!
I hope all above helped you make a sound decision for your betta.