Like so many things said and done in the GF, this one is up for grabs. I believe it is fish food. In the scene, Vito is older, sicker, and Mike is running things. Like in the novel, as Mama Corleone says, "he plays the fool". He is almost absentmindedly concerned with the fish. We never actually see him feeding the fish, only doing something with his hand over the top of the aquarium. He does turn around and pick up what I think is the fish food box from the desk because that's probably where he left it the last time he fed the fish. When he does pick up the box he appears to be expresiionless and reading the box's label just like anyone would who is older. He doesn't feed the fish from the box, but that is probably because that was edited out. If you notice, when the camera does swing back to Vito, he has moved to another part of the room and is pouring water from one container to another. So, obviously, some editing did take place.

I don't think he's holding a radio. Indeed, what would he be listening to and why would he be staring at it like he does? You can't see any dials as you would expect to if it were a radio.

However, if you look real close at the fish food box, you can read the label: Brasi Bits.