Somebody somewhere was suggesting the use of plastic clothespegs, although I'm darned if I remember who or on which forum here.
But I thought that was brilliant.
Just have to try finding some, so no more tracking of separated rocks, elastics and free-floating veg in several different tanks every time...
i use something I bought from Ebay called a Screwcumber. It looks like a stainless steel coil which you can push through any fruit or veg and it will keep it on the bottom. you can buy one here