Koi not happy in new home.

Well it's mid July and I thought it was about time the koi moved outside for some fresh air. I have used a stock tank for the last few years. This year it finally sprang a leak, so I lined it with 6 ml black sheeting.
Apparently they have changed the plastic as it seems to be leaching something in the water. Two days after the move, they were all gasping at the top, despite a good filter and aeration system. I did a water change, but when I go thome the next night, my 1 foot + koi was gasping his last. He was too far gone. I blamed my husband for not seeing they were struggling and adding water and blaming my Dad, because I had to
cook for him and play cribbage, which if I hadn't had to do that I would have been home to save him. Sounds justified right This morning the stock tank was half empty, so the liner leakes. Tonight they will go back inside where they are safe. I have a huge hole in my yard for the "new" pond, but I am getting very depressed thinking I won't be able to get it up and running properly. Anyone know of a use for a huge hole in one's front yard, if the pond thing fails again?