Thanks for all the help. Here is what I just did:

Toilet tank is part of a one piece toilet, and not leaking badly, and the crack was on the back (looks like it got banged) and not in a stress point like on a bolt. Nothing too critical underneath in the basement. And the leak has been here since bath remodeled a year ago with no change.

Drained and dried with hair dryer.

Used a little purple PVC primer to try to see the crack better. Little help.

Put a layer of epoxy with Q-tip.

Cut a few strips of "sheer panel" about half inch wide. (Sheer panel is nylon used in draperies, get lots at 2nd hand stores. Other choices would be nylon window screen, fiber glass, etc. Anything that is strong and won't rot.)

Stuck the sheer panel strips to the epoxy, then more epoxy on top, all with Q-tips.

Done and doner.

How has it worked? Has it cracked again?

Here you go !

Score or widen the crack to aid in adhesion and apply hydraulic cement.

Since the tank is a hairline crack on the bottom, repairs will not bother the ascetics and you can avoid buying a whole new unit. I recommend using Heavy Duty Job Site Locktite Epoxy. Get the 8oz version, it's water resistant and comes in a 2 part mixture of resin and hardener. You can remove the tank and it's components in 5 minutes. Then, properly dry everything inside and out. Clean the area you want to repair and apply a line across the cracked area both inside the tank and out. It says it drys in 5 minutes but wait a full 24 hours to cure before you reassemble your tank. Since everything is off, you might as well grab a tank to bowl kit if the toilet is at least 5 years old and replace the rings to prevent future leaks. If you want to buy a new toilet, I recommend your local hardware store or plumbing outlet. Tanks aren't typically sold separately so you're looking at a new complete unit if you go that route.

Hi
I came across your posting today while I was looking for a solution to fix the cracked toilet tank.
I would prefer to fix it rather than changing the whole toilet.
Can you please let me know if the JB weld worked?
I know it has been few years now but would really appreciate your reply!

Marine epoxy is better. Tank must be very dry. Some people also imbed a piece of vinyl pool liner in the epoxy over the crack. May work for a while, may not.

Years ago when I was too poor to afford a toilet replacement, I used JB weld as well, worked like a charm. Was still holding up when I moved several years later. Only problem was the crack was a little more visible, since the JB weld was dark. Don't know if they offer a lighter colored version, or maybe caulk or something over top of it, if that's a problem for the customer.

I have found that most of the cracks in the toilet tank come from faulty installation. Let me explain. One of the more common mistakes in installing a toilet tank is to crank down hard on the tank bolts. If a person leans back while sitting on a toilet the tank has no give and will crack around the tank bolts. The correct way is to allow the tank to rock a quarter of an inch. I do not recommend repairing a cracked tank. However, if you want to try I can only echo Brian's words. May work for a while, may not periods. Good luck, Tom