Why did my rainbow trout die?

I live in australia and i bought 15 rainbow trout for a garden set up. The temp in the tank was 13C, The pH levels were 8.5 and they had lots of food. I let their container sit in the water to ajust to the temp for an hour, then i put them in one at a time. About 2 minutes after each fish was put in the tank they spazzed out and died. someone help me?