You can either tie it to something, or do what I do. I peal it, cut it into strips, and boil it for a very short time. Don't boil it long enough to make it a gooey mess, just long enough to soften it up a bit. After that, it does sink all on it's own.
I've also read that you can't over feed your fish when feeding them veggies. I don't remember the source, but I do remember it was on a site that is rather knowledgeable.
-Phroge