That's the second smaller pond where the water will be pumped into then back into the main pond via water fall/stream/rockery.
The smaller pond is going to have lots of vegetation to aid filtration
Your second smaller pond is the perfect candidate for a bog filter. Have you considered that? Have you read about the success that many members here have had with bogs?
My pond is over 10 years old and I've struggled with trying to obtain clear water. I've tried homemade filters, pressure filters, UV lights and never had clear water.
This Spring I build a bog and within a week my water went from completely dense green to clear. In the following weeks the water became literally crystal clear. Not only am I able to see very bottom (3 feet down), but I've spotted tiny black fry that were no more than 1/4" long. I would have never been able to see that well in the past.
The beauty of the bog is zero maintenance. No rinsing of filter pads, no backwashing. The only thing that has to be done is the possible thinning of aggressive plants that may take over the bog.
I sold my two pressure filters and UV light.
I'm so pleased at the results of the exclusive filtration from my bog. It has been, without a doubt, the utmost best improvement I have ever done to my pond.
If you need more information on bog building, we can help. Just ask...