Stress is a big killer of new fish. Acclimation is a good way of helping reduce that stress, so Speed's question remains... Did you at least float the bag in the tank water before netting them out of the bag and puting them in the tank? Some people prefer the drip method, adding water slowly to the water from new fish's bag over the course of a couple of hours.
The new tank will need to be cycled, but if you are shutting down the old tank and completely moving over to the new one, you could just move the filter from the old tank to the new one, and it would act almost like an instant cycle. Be sure to use a good dechlorinator in all water (Prime is a preferred product). Do you have a test kit? Avoid the test strips, and be sure to use a liquid reagent test kit, like API's Master Test Kit.
If you are not completely shutting down the old tank, I would suggest taking some of the filter media from the old tank and puting it directly in the filter of the new tank (it doesn't have to fit nicely, just cram it in there) until you can be sure that the ammonia and Nitrite levels are down to zero. This may take a day or two of testing the water to make sure.
There is always the chance that you got sick fish too. Try to avoid adding additional stress to the fish if you can. If they appear stressed, don't move them to the new tank, as this can push them over the edge.
