There's a dearth of articles on how to fix a leak in a large plastic pond. It took me quite a long time on the Internet, talking to pond professionals. Frankly, I was surprised at how long it took me to find out.

Instructions
Things You'll Need:

* Cheap plastic container.
* Bike tire repair kit
* Eye dropper
* Nylon Scour pad
* Milk

1.
Step 1

Most Important! Your water loss may be evaporation. Take a cheap plastic jug and fill it with water. Place it on a ledge or secure it level. Cut the top of the jug so that the water in it is at the pool level. Wait for a few days. If the pond at jug levels stay the same, you don't have a leak - it's evaporation.
2.
Step 2

Okay - you have a leak. get an idea of how much you're losing per day. Now cut off your water circulation for a few days. If the leak rate stops? The leak is in your pump.
3.
Step 3

You keep losing water. Now? You must leave the pond alone until the water level stops dropping. If you have Koi in there and it keeps going? You might want to get them a new temporary home.
4.
Step 4

The water level has stayed at the same level for a few days. FILL THE POND WITH WATER - BUT ABOUT A QUARTER INCH ONLY! Put some milk in an eye dropper - then after you've stopped the water and let it calm down, go around the pond - on your hands and knees, dropping milk in around the side. Watch for any turbulence - then you'll know where the leak is. When you find the leak? Don't stop, there may be more.
5.
Step 5

Once you've found the leak, let the water go below the level it's at - this may take a few days. Scrub the area around the leak with a nylon scour pad. Then use the rubber cement and patch fromn the tire repair kit. let it dry for an hour or so - then fill the pond again.