Q: Is it okay to put colored marbles in a bowl/jar with a betta?

Question submitted by Kimberly, Wakefield - Massachusetts
A: The 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.
bulletgravel can make it harder to clean the jar. The more work it is to clean a jar, the less you are likely to want to do it. Hence your betta will suffer from lack of frequent water changes.
bulletif you feed live food, they will hide in coarse gravel/marbles, making it hard for your betta to see them and eat them
bulletuneaten dry food can also fall between the gravel making it hard for you to see that jar should be cleaned asap. 
bulletornaments that have holes in them are a hazard to bettas. Bettas are very curious and will get their heads stuck in there. Once the gills are stuck, they act as an anchor, there is no unstucking Mr. Betta :(((.
bulletuse only gravel or ornaments purchased at the fish store. Rocks you find while hiking, your little brother's marbles, sea shells are NOT suitable ornament for a betta jar. Why? Because rocks can be contaminated, or dissolve and contain harmful metals etc, little brother's marbles may have been up his nose LOL. But seriously, they might also be contaminated. Sea shells or coral may cause the PH of the water to JUMP and will hurt the betta. Pet store ornaments are sterile and made of material that have been tested and are safe to fish.

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.