Hi my first post so hope someone can help, I have set up a fibreglass pond for my wife's ducks. She loves them to have lovely clean water so it gets emptied often. To save doing this with buckets I purchased a pump and connected this as an inline pump (as it stated it could be set up on the box),the pump struggles to. Empty the pond or to start once going its fine. Help required what am I doing wrong.

The pond is sunk into the ground approx 1 foot then it. Has approx 8 inches above ground. I have drilled a plug hole at lowest point connected the hose which in turn goes to the pump and the outlet pumps direct into a drainage pipe approx 8 inches below ground. The pump sits on the ground but states it can pump 6500 litres an hour and up to a height of approx 4 metres. I really didn't want to put the pump in the pond as the ducks love to dive and swim under water (just had thoughts of one getting stuck in the pipes) if I have to then I will but I feel more advice is required.

Sorry for the very long first post
Cliff