Tank not sealing

We recently replaced our toilets. One tank is not sealing. Its an 18 inch toilet. The tank sets right against the wall. Any ideas on how to get it to seal? The toilet is an Aquasource. I don't want to have to buy a new toilet if possible. We tried to tighten nuts/bolts.
Thanks
D

Where exactly, is it leaking from? A tank often has 4-5 holes in it: where the fill valve is attached, where the water goes to create the flush, and 2 or 3 holes holding the tank to the bowl.

It's leaking where the fill valve is. Then when we took it apart and tried again...
It's leaking from the fill valve and the bolt holes. Afraid to continue to tighten because I don't want to cracker the tank.

Take both the fill valve and the tank bolts out, use some fine sand or emery paper and lightly sand the porcelain until it is smooth. This allows the gasket to seal better. THen, reinstall the bits. On the tank bolts...it works best if you have two sets of washers and nuts. On the tank bolts, only use the rubber one inside the tank...IOW, the ONLY thing under the head of the bolts should be the rubber washers. It works best if you then put a metal washer under the tank and lock it down with a nut to make the seal. Then, use the second nut and washer to hold the tank to the bowl.

Are you sure it isn't the spud washer? I can't remember the models listed on the packaging but if the Aquasource has a 3" valve then the Korky 481BP may work. If the leak is at the fill valve then that should be easy enough to remedy by installing a new fill valve.