How can I repair a toilet that leaks at the tank/bowl connection?
The washers inside the tank can leak, and the body of the valve that empties the tank to bowl can crack, making tightening the gasket pointless. There is a nice set of tank to toilet bolts on Amazon that are solid brass, and have extra nuts and washers so that you can seal the upper tank independently of squeezing the tank to bowl connection.

You can get very affordable toilet rebuild kits that supply all of the necessary hardware to eliminate all leaks except for the bowl to floor connection.