Silicone. Comes in a squeeze tube at the hardware store. Put some on every connection between the tubing and some other part. Clean-up when still gooey is a damp paper towel. Good luck.
Vanessa

how do I put a picture on a post? It's only the basin(tub) that's leaking..I would have to cover the whole thing in silicone? The tubing seems to be fine. Thanks alot for your reply.

Oh no, not the whole thing. If the basin is leaking rather than some point where the tubing passes through the fountain, there must be a crack which you'll have to find. That's trickier. There are a couple other reasons you could be losing water, too. A picture might really help.

A lot of members here use photobucket which is nicely set up for posting pictures. Do a search on this forum and you'll find better instructions than I could give you. It's free, BTW.

Diane, I have two bird baths that are leaking from cracks. It must be cracks because the water 'evaporates' too fast! I can fix them for a summer or so at a time with caulking from the hardware store. Silicone is one kind, but there are other kinds, so read the labels to find the one that works for the polyresin. I rub the caulking down into where I think the crack is, with plastic gloves on, because it's hard to get off your hands. Then let it dry for a few days. My birdbaths are concrete, and the concrete fix stuff is latexy and peels loose after being in the hot sun for weeks. So I do it over in fall or spring. I hope it stickes better on your resin!

:) Mary

Thanks so much for your help. I have a thought..this basin is so large..I wonder if I could use a liner in it..instead of sealing it. What are your thoughts on that???

Liner is a possibility, it would eliminate the chances of a crack. Without a picture I can't be sure, but a lot of times its just splash and evaporation. Keep looking for cracks, but if you can't find any or when the liner is in and it still 'leaks' its probably just normal water loss. Most resin fountains aren't big enough to catch all the splash. A rough rule of thumb is for the basin to be twice as wide as the fall is tall. Good Luck.