My issue is what is best to fill around my pond. Last fall we got over run by rats for some reason so I had to pull out the prefab pond which was 4 foot wide by 8 feet. The rats had made tunnels around the pond and we needed to get them rid of. We had an exterminator come in and within 3 weeks the rats were gone. Under the plastic prefab pond were holes and tunnels every where so I removed everything and leveled out the area. I will be ready to reset and level the pond again soon but what is best to fill around the pond so that the rats do not stay and make their homes if they come back. I was reading to place sand on the bottom to level off the pond again. Then fill around the pond with a small rounded stone or type of gravel That way any digging then the holes will cave in and the rats will move on. Any suggestion will certainly give me a helping hand.