True, the pH will likely shift upwards gradually and the danios will be able to adapt, but if you are going to use this product for the arrivial of your danios then just use it that one time, it will not be necessary anymore after that. Besides why would you need to use it if your pH is testing at 7.0-7.2? Don't worry about your ammonia testing at 0 since it is most likely accurate because your fish were only dead in your tank for 20 minutes. 30 minutes of them being in the tank alive and 20 minutes of them being in there dead is not enough time for ammonia to accumulate in the water. Hmmm...."I can add the stuff while they are in there and it won't harm them", what do you mean by this? Do you mean that your are going to add the "pH Down" to the water in the tank while the fish are in there? If this is the case then you are wrong, it will harm them. If you are going to add this product to the water than you need to do it at least an hour before you put the fish in.