For Dendrobates, thumbs and Oophaga I would not worry at all, they are adapted to eat tiny prey captured with the tongue, a chunk of aquarium gravel is not likely to stick and would be spit out immediately if it did. It may be more of a concern with dart that will take larger prey in the jaws like Phyllobates, Epipedobates, Ameerega, etc. though I would still be surprised to hear of it happening even with them.