I have a small water tank with few fish at home. There are small stones for decoration, and for several weeks now, I noticed that every time I'm around, the fish go down to the stones and bump into them on purpose, and it results in some noise.
They didn't do it from the beginning (bought them in August 2019), just maybe 6-8 weeks. (Literally)
They do it now consistently, so I can't help wondering: is it their way to draw attention? I walk to them when they do it to check what they do then they swim up towards me.
To make it more clear, here is the fish tank with the small stones: