Hi, Once terracotta starts to break down, it's very difficult to fix because there may be microscopic cracks and water will always find a way to enlarge these fissures.

However something that you can still do is to use an aquarium silicon sealant to help seal the cracks. You can also paint to seal the interiors preventing water from entering the terracotta. Terracotta is by nature very porous and so its important to waterproof them when using them to hold water.

Ianna

One might also consider looking for a plastic pot/liner that will slip inside the terracotta one. It is very easy to patch plastic pot holes to make them waterproof, so if you were lucky enough to find a good match, you'd be in business.
Good luck,
Jan

i have posted before about coating glazed urns inside with roofing tar-to make waterproof.i think you can use the same method for terracotta.you can read about my method at www.cottonarboretum.blogspot.com OR see below:

Making glazed pots into FOUNTAINS, and cleaning them
Posted by arbo_retum z5 ,WinchstrMA (My Page) on Mon, May 1, 06 at 14:38
SEALING/WATERPROOFING THE POT:
We have converted three large chinese red glazed and blue glazed earthenware pots- to fountains. we have plugged up their bottom drainage holes by putting black duct tape underneath the hole, and then from the inside of the pot, filling the hole with 'Bondo'. after the Bondo has dried, we apply a thin opaque coat of roofing tar to the entire inside of the pot, up to the glaze. once that has dried thoroughly, the pot is ready to put in place,fill with water , and insert a pump, which we put on bricks.over the pump's water 'nozzle', we then put a piece of tight-fitting black pvc pipe , cut long enough to make a nice fountain but not too high or low. (this height issue and the water fountain 'look' that you want are what takes all the tweaking.)you may also want to play with various pump sizes.
ELECTRICITY:
even though electricians don't recommend this, we then plug in the pump to an electrical outlet using outdoor green electrical wire- like you use for outdoor xmas tree light.we don't bother to bury the wire(unless it crosses a path); we put mulch/shredded leaves over it to hide it as needed, and we do not take it up for the winter. the one drawback to this system, where we plug up the hole, is that you can see the electrical wire that hangs outside the pot, but that can sometimes be camouflaged. we use sealed pots for fountains but also for growing waterplants (cannas, elephant ears, water iris, waterlilies).
we bring in our pots onto a screened porch but in z.5 they can be turned over and left outside if they are raised up off the ground.

CLEANING:
the ugly white milky lime deposits we eventually get on the outsides of the pots-i clean in the fall or spring.we tried Lime-Away but it didn't work well. Instead i devised this system: I use XXXX grade-the finest- steel wool dipped in BOILING vinegar(i plug in an electrical fondue pot next to the pots i'm cleaning to keep the vinegar boiling!!)I go over the outside glaze once-gently but firmly, keeping my st.wool very wet w/ the boiling vinegar. then i let the pot dry. if it still has lime, i go over it again and let it dry. then
i coat the pot with a thin coat or 2- of butchers wax, buffed with towelling.

the pots are placed on columns/pedestals in the gardens.
i personally feel every garden needs some form of water. what a difference it makes.our gardens are on a busy main street so we have fountains and waterfalls spread throughout the garden. the sound is soothing and inviting and helps mask the street noise. i hope some of you find this info useful!
best, mindy

I put together a terrracotta fountain this weekend and I have 2 problems. The hole in the bottom bowl is leaking. I sealed the hole inside the bowl by attaching a plastic saucer using "Liquid Nails" but did not seal it on the outside of the bowl. I read the suggestion of using epoxy and plan on using it to plug the outside of the hole. Should I plug it with something first like a cork? Should I take the entire fountain apart and reseal the the hole from the inside too?
2nd problem, the water bubbles up. I want it to trickle or spray over the sides of the terracotta vase into the bottom bowl. The vase is topped off with a terracotta saucer, the tubing sticks out the center hole of the saucer and is topped off with a copper flare. The pump is a medium Beckett fountain pump for fountains up to 3 ft. The vase is 3-1/2 ft. To get the affect that I want, do I need a different pump or can I attach something to the top of my tubing?

I've been searching for a product to waterproof a decorative terra cotta pot for a deck water garden.

Question: would either the roof tar or epoxy approach be toxic to goldfish?

Yes for sure on tar. Epoxy depends.

My son threw a fire cracker in the base of my fountain which is a terracotta pot. It ended up in three pieces. DH use epoxy to piece it back together. It's held up for years.