Hi nugie. Had that problem when i adopted my 3-inch kois and placed them in the pond with more-than-a-foot long kois. The bigger kois were so nasty I thought they thought that the new ones were part of the food chain! Anyway, I was worried so I looked all over the place and found a small dog cage. Placed the cage in the pond and whoa, as expected, the smaller kois found a safe place to go in while the bigger ones just swim by. I could sleep that night. The next day, I see the smaller ones getting bolder and when they felt bullied just hurried back to the cage. I think it took 3 days for them to finally get used to each other. Hope this helps.