Back to the OP. It really couldn't hurt to try with a chunk of uncooked potato. Cooked potato seems like it would be way to easy to get muchy and pollute the tank. Fish have very sesitive chemical receptors and are very unlikely to poison themselves with any natural food. Fish eat fruit/veggies that fall or get washing into their bodies of water all the time in the wild. The fish may ignore it, however, so do be ready to pull it out before it messes the tank. I do this will all the fresh veggies I buy, I give the fish a small slice and see if they take to it. No small peice of food will seriously foul the tank.
One final thing, however, the eyes of the potato do have some toxins in them -- there are reports of very young children becoming sick from mashed potatoes without the eyes cut off -- so, I wouldn't use a piece of the potato with an eye on it.