I think it's the rocks.
If all these rocks are identical, take one of them out of the aquarium, put few drops of vinegar on them and see if they make any bubbles, sound or smell. If they're different rocks then do this test on all of them. Make sure to rinse them with water before putting them back in the aquarium (if the test showed they're not reactive).
Waiting your post!
If all these rocks are identical, take one of them out of the aquarium, put few drops of vinegar on them and see if they make any bubbles, sound or smell. If they're different rocks then do this test on all of them. Make sure to rinse them with water before putting them back in the aquarium (if the test showed they're not reactive).
Waiting your post!