I have to agree, Bag acclimating is okay for some fish but more sensitive fish, such as tangs, it is not a good practice and may be the cause of the outcome. Ich, is an indicator that the fish was stressed and unable to fend off the infestation. All fish carry parasites so it's the stress level that does them in and determines if they get sick. By bucket acclimating over a longer period of time, the fish gets to slowly adjust to the new water parameters in your tank and makes a less stressful transfer. The length of time needed is determined by the volume of water the fish is started in. The object is to go from 100% bag water to approx. 5% bag water and 95% tank water.
As for the eco system in the bucket, since you are adding new water continuously, this is not a real concern. What does need to be watched is the oxygen level of the water and that can be remedied simply by adding an airstone in the bucket ( from the start) with a slow setting to not over aerate the water. Your fish should look "normal" with normal respirations while in a bucket being acclimated. A drip of approx 1 drop/ second is an average rate to use.
Hope this helps