I wouldn't leave it whole unless you actually need that much material and have a spot for it in the tank where it won't be shading plants. It does take a while to get soft enough for them to go for, so I usually have a few large-ish pieces in the tank of different ages that I rotate. I put the new ones in the back and then pull the juiciest one in the front of the tank so I can watch them graze. When that piece is mostly gone I pull it out, move another piece to the front and add a new piece in the back. (I should note that I am not trying to alter my water chemistry with the leaves - I am not sure how much material you would need to do that, but it's more than I use.)