I replaced the flapper in my ceramic toilet tank and the leak from the tank into the bowl slowed, but it still leaks. What is the next step I should take to get rid of this leak?
Make sure the gasket that the flapper rests on is perfectly clean. Any grit or sediment on it can prevent the flapper from making a good seal.
Check the chain that runs from the handle assembly to the flapper. If it's taut when the flapper is down, it may be holding the flapper slightly away from the gasket, again preventing a good seal.
If that doesn't work, it may be time to replace the whole flush assembly
If just replacing the flapper was insufficient, you'll need to replace the entire flush valve assembly. Replacement kits like this can be found at Lowes or Home Depot
I resolved the leak by moving the rubber hose that brings water into the tank that was feeding into the cylinder in the center of the tank to which the flapper is affixed at its base. The hose was down inside the cylinder and I found a plastic clip at the top of the cylinder that had a nib for the rubber hose to attach onto. With this change, the water still goes into the cylinder, but now it is up in the air, so their can be no siphon effect. I'm guessing that the leak was somehow related to this since with this change, the intermittent slow leak ceased.