Why did my tadpoles die?

About three or four weeks ago, I brought some tadpoles from a near-by pond in Wisconsin. I was keeping them in a plastic tupper-ware box and I put distilled water into the container, then put the tadpoles into the container. A few minutes ago, I was changing the water (i would change it every 1 or 2 days depending on how dirty the water got) and i tried a new idea of changing the water since it was faster. For the past few weeks, i was taking the tadpoles out one-by-one using a plastic spoon but this time i pored them into a strainer, refiled their container with distilled water, then pored them back in quickly. Only for out of the 10 to 15 survived and the ones that didnt survive died almost instantly. What happened? Why did they die? (the distilled water that I was using was kept in the same room that i kept the tadpoles in so the water tempature was closly the same). Also, I was feeding them lettuice that i boiled in the microwave in distilled water (none of them died before this).