How can I eliminate condensation on my toilet tank?
When warm moist air comes in contact with a cold surface condensation forms.

There used to be these tank liners sold that you would install on the inside of the tank to insulate it. It keeps the outside of the tank from getting as cold to reduce or eliminate condensation. I did a quick search and couldn't find it anymore.

I did however find toilet tanks that come with insulation already installed. See http://www.homedepot.com/p/KOHLER-Memoirs-Classic-1-28-GPF-Toilet-Tank-Only-with-Insuliner-Tank-Only-Liner-in-White-K-4433-U-0/203006152#

The liners were just sheets of styrofoam. I guess you could get some styrofoam and cut it to fit. You'd have to cut the kits anyway. Then glue it to the side and bottom of the tank. Going with a tank that's already insulated would be a lot easier.