Unless your water is ice cold and your house is pretty warm, it won't be condensation.
What type of filter do you have? It could be leaking for a number of reasons, I would test that first, if it's not leaking then I would return the fish, or find other suitable temporary housing for them and empty your tank, take it outside and fill it up with water and watch it. If it is leaking go get some DAP household silicone. It says aquarium safe on the packaging and has a picture of a fish in the bottom left hand corner.DAP Products - Silicone Sealants - DAP® Household/Aquarium Adhesive Sealant
You can purchase this at HD or Lowes for a few bucks. Let it sit for 24 hours then fill it back up outside to make sure, so you don't have to tear down your tank again.