If you have an early generation Aquascape skimmer ( as shown in photo and video) and have been loosing water, you may have a leak at the skimmer. I have experienced a number of skimmer fasteners that corrode to the point that the head is dissolved completely and the fasterners below the water line are no longer securing the skimmer faceplate. This has resulted in water leaking at the skimmer face plate. If you have a pond leak with an early generation Aquascape skimmer, you can perform a quick test as follows: Fill the pond to the usual water level, and without the pump on or water flowing, observe if the water drops down to the lower area of the skimmer faceplate or just below the faceplate. This is an indication that you may have a leak at the faceplate and if you are not up to repairing it yourself, you may want to call an experienced pond contractor that is familiar with such repairs. This may require a new skimmer, but I have been successful with drilling out the old fasteners, sometimes tapping them again, and installing new fasteners ( Stainless Steel). Sometimes the lower fasteners have to be drilled out completely and new bolts and nuts installed to secure the faceplate. Don't forget the silicone between the skimmer and the liner.
I have also seen situations where the liner has been stretched and wrinkled so badly that securing the liner again can be challenging. This is where an experienced professional can help and it may involve cutting away the badly distorted liner and gluing a new piece to be secured to the skimmer.