I attempted to fix several cracks in my Rubbermaid stocktanks a few years ago. It almost worked, but the pressure of 100 gallons of water on it was too much. But it might work for your pot.
I forget the name of the stuff. Horton told me about it. It comes in a big syringe. Why don't you post a note addressed to Horton and he might be able to help.

oh man water pressure! I forgot about that doh! thanks I will. Which just made me think what about a pool or hottub repair product....

How big is your bowl and is it ceramic? I'm wondering if you could ask at a nursery. But you do have to be careful about your fish.
I'm also wondering if you could find a couple watergardening sites online that maybe sell these bowls and they could advise you on how to fix it?

Is it glazed or terracotta. Can the crack be hidden? Once repair stuff is cured it is not likely to harm your fish but the label will tell you. There are epoxy sealers that are transparent and can seal a wall that has water shooting out from underground water pressure.. They are used to seal basement walls. It is expensive though and it might be less costly to buy a new bowl.

The bowl is a weird thing its sorta a concrete ceramic, non glazed. It must have had some sort of water proofing I would assume. I read people use a tar product and it did look like something was painted inside. Not a thick tar but a thin matte coating of some kind. Its about 2 1/2 feet tall and same wide. Right I forgot about that paint stuff! You think the store that selling 100's of them would have some idea but they seemed clueless. I could put the paint stripe/seal to the back. There about 180 new. This is it actually, the last post, big one in the middle. http://forums.gardenweb.com/forums/load/cottage/msg0610430425222.html?3 Do you think I would have to do the inside and the outside?

The walls of that pot are pretty thin to support much water. The only way I can think of would be to coat the entire inside with the Epoxy paint to essentially create a pot within a pot. Otherwise, the water pressure on parts not covered would force the edges of the crack to separate. The Epoxy would not be damaged but the clay would crumble and split. Most Vietnamese pots are for plants and are fired at fairly low temperatures which means they are not particularly strong and water soaks into and through the walls like a sponge. Changing temps cause the water to expand and crack the clay. That is why they are coated on the inside with tar. It makes them waterproof. If the coating on the inside was not black I suspect it might have been latex paint which would be a lot cheaper and thinner and less weight to ship. It is also more likely to let water through.

Pots from Mexico are fired similarly to the Vietnamese pots and are just as fragile but not quite as expensive. Pots from Italy are much more durable since they are fired at a higher temperature, making them harder. The kind of clay makes a difference as well. The pots I lust for are made in the southeastern USA. but I can't afford them.

Other than buying a new pot, you might look for a plastic nursery pot to set inside the clay pot. That would remove all side pressure from the clay pot. You would have to seal up any drainage holes with Great Stuff. Perhaps one of the small pond or barrel liners would fit.

wow thanks! ya I bet it is latex then I didnt think it was tar but wasnt sure what it could be. Its sold as a 'water bowl' Its pretty thick id say and inch and half. I bought some elmers silicone sealer, nontoxic, uv resti, waterproof.. Then I found foundation sealer 'can stop running water' Sounds fancy. I havet seen great stuff around yet. Ya I was thinking if I had to a pond liner might be my only choice. Thanks for the great info. What do you think about the silicone? Id just run a big fat bead.

I use Plasti Dip ( a liquid latex for dipping tool handles) available at home depot/hardware stores. After it cures it is non toxic. Have used for large Italian clay pots to grow water lilies (outside of the pond sitting on the terrace) and for discussing plumbing, cinderblocks, etc in the pond. Very water proof (seals terra cotta to hold water), ay take a few coats. Comes in colors (I use black) and in spray or liquid. Down side is once open has a very limited shelf life.

I was gonna suggest pond foam, or aquarium sealer.

There are liquid 2 part epoxys that will penetrate into the crack and seal it as well as gluing it together even stronger than the ceramic. Once cured, they will not be toxic to fish.

Aquarium silicone sealer.