I could be mistaken, but I believe those are not marbles. They are actually fairly big. There should be no chance of any siz frog accidently eating those. I agree about the cave though. Over the years of fish keeping (28) I have had just about every tragedy happen. I don't use anything that a creature could get trapped in or under anymore. I try to think of the worse case senario before adding something to a tank. With frogs there are a few additional challanges that you don't face even with fish.