I agree. I think I'd recommend putting coconut shells in there instead, it's safer. Coconut shells must be stripped of the hairy parts and the inner white meat of the fruit I hear, then boiled to leach out the tannins.it does look like they are.... if they are i would remove them as they may contaminate the water if the print comes off them and i doubt they are made of aquarium safe plastic either just to be safe
you can always make some caves from rocks or wood etc.