Why does my tank smell so bad?

I own a gerbil, and today i noticed that a corner of his tank is soaked with, well, something. i dont think that it is from the water bottle leaking, because it is on the other side. But, he does push the bedding around the waterbottle into the corner. The water bottle is Oasis Brand, and the most i have seen it leak is basically a drop or two. I'm not sure if he is peeing all of it, but the tank smells NASTY. What could it be and how to prevent it?