Over time, you will need to "top off" the water in your underground basin. However, if the water is running out within a few hours there's a bigger problem. Usually the reason is that the splash of your fountain is going OUTSIDE of the diameter of your underground basin. The water needs to trickle down the rocks and go back into the basin for this to work properly. First, try positioning your rocks in a way that funnels the water back into the center of the basin. If you want a larger splashing fountain you may need to invest in a large and WIDE basin (or use pond liner as mentioned above).