I have a medium sized pond and everything has been fine until recently. A large number of my fish have been dying in last 2 weeks (no obvious marks on them). New changes to the pond have been:

1) extended the pond, new liner (2 months ago)
2) Just introduced 7 small koi into pond (3 weeks ago)
3) New plants (water lilly's, 4 weeks ago)

I'm going to check the water chemistry but I've had no problems in the past. Can anyone help?