Okay, here is a little test you can try (if you have a closed top system) and to see if its condensation or if its actually leaking... although i'll admit it has its flaws...
1) you want to find out exactly which corner you suspect is leaking and dry off the outside ares of the tank, floor, stand etc. then get some tissue and fold it over so its a thick strip
2) tape one strip to the top corner of the tank, one in the middle, and one on the bottom corner of the suspected area of the tank...
3) check back every 5 - 10 mins to see which piece of tissue is starting to get wet first... if the bottom one is getting soaked, chances are you may have a leak... unless the water is trapped between the tank and the stand its on...
I only say to do this coz i had a small leak with my 200G when i first brought it home, then after re-sealing it i noticed what i thought was a 2nd leak just to later find out the it wasn't a leak, but condensation which was running down from the back of my tank into the same corner which was leaking the first time around...
Anyway, best of luck with your tank, and hopefully its just condensation and not actually leaking...