Just a wild guess, but maybe the tank isn't cracked, but the toilet is.

Alternatively, if it's a new toilet, maybe either the tank or the toilet has an imperfectly shaped hole. I wonder if a bit of plumber's grease smeared on the gasket would help.

Maybe you also need to check the bolts and their washers...or at the water connection.

I suspect it may not be a leak at all. When the toilet if flushed the tank is immediately refilled with cold water which in turn dramtically drops the external temperature of the tank and there is a strong likelyhood that what you are seeing is condensate water forming on the exterior of the cold tank. As the toilet sits idle the water in the tank warms to room temperature and the formation of condensate stops leaving the illusion that the moisture is a result of a tank leak.

Pinoke where are you?

I am no expert, but I think you need to see a proctologist for a leaky bowel. At least take some Immodium. I wouldnt want my plumber touching my bowel. You state that your bowel is bone dry in the morning, well I find that usually my bowel wants to leak in the morning, very interesting...

OH!!! I get it, you mean bowl, not bowel. Now it makes sense. That I cant help you with.

Are you using the correct parts?

I recently fixed my MIL's tank that leaked every time you flushed it. Her's needed a new gasket that goes between the tank and the bowl. I noticed while at Lowe's that there are at LEAST two different type. There was the (thin) "universal" gasket, and an "American Standard" gasket. I happened to recall seeing the "American Standard" symbol on her tank while taking it apart and managed to get the right part the first time (wow). The "American Standard" gasket was MUCH taller than the "universal" gasket. I believe had I tried to use the "universal" gasket, the thing would have continued to leak.

However, if you actually paid a plumber to fix it, I would have thought he would be experienced enough to insure that he wasn't making a silly mistake like using the wrong part.

I seem to have this problem in one of our bathrooms. That is, there is a leak at the bottom of the tank everytime I flush. How can this be fixed? Thanks, Ravi

Needs a new gasket. Will require removing the tank from the base and taking the old gasket to the store to get the proper replacement. If you unsure how to disassemble without flooding your bathroom - hire a plumber

Hubby replaced all the black rubber gaskets on our Universal Rundle 2 piece toilet. I bought it when low flows were scoffed at, especially by plumbers, when I visited plumbing supply places. We installed THE UR ATLAS model ourselves (we are mostly DI yourselfers )new back in 1992, low flow from Lowes. I had some parts in a bag I assume they were from when ordered from UR back in 1995. when replaced fill tube. I keep owners manuals and most receipts in binders and later on a description of problems and how solved.
This time water was black and black rubber really bad deteriorating. Had a package of universal bolts ( pack new bought years ago from Furrows). Hubby took tank outside and nicely scrubbed out black and installed all new Gaskets that I had. He bought a new Korky flapper from Lowes, a universal kind Bolts rusted and caused a divet in the porcaelion or china inside of tank near bolt openings. A little later, he noticed water on the floor so he put in some silicone and plumbers putty after draining again and wiping dry. Still leaking some around bolts. dripping slowly to floor. Course, Lowes didn't have any parts for UR. I forgot to take my owner/parts manual.
HOW CRITICAL are the real deal mfgs bolts ??? Those universal pack from the 1990s don't look as tall or wide as what we took out
Im not sure if they are still in business..... Universal Rundle that is. Will call myself tomorrow the phone number I have written on booklet
Thanks all
Jean

Installed new gasket on tank but still leaks when flushed at full water capacity. Does not leak at half tank capacity. Any suggestions?