check this out

from
http://swimming.about.com/od/pooland...leak_tests.htm

Excessive Splash Out or Evaporation
Lets touch on the splash out and evaporation issue first and see if it
can quickly be eliminated.



If the pool is not being used frequently (then obviously), it is
unlikely that the problem is splash out. On the other hand, if it is
the middle of the Summer, with high temperatures and lots of kids
getting in and out (dripping wet) this could, in fact, be a real cause.

To eliminate evaporation or splash out pool water loss as a causes,
here is a quick test that can be run on your swimming pool:

* Place a 5 gallon bucket on the second step or bench seat and fill
it with water so that it is exactly the same level as the swim pool.
* If evaporation is the cause of your loss, both levels should drop
at the same rate.
* If the bucket drops faster, you probably have a thirsty dog - so
be sure to secure all pets BEFORE the test.
* If you have a pool leak, then the pool will drop faster than the
water level in the bucket.

Once you figure out if you are actually losing water in the pool via a
leak, and that is not just evaporating or splashing out, there are a
few more tests that can done - but now might be a great time to call in
a pool professional to evaluate the situation.