|This is a normal response of the biological filtration when adding new livestock. The amount of ammonia being produced by the new inhabitants is greater than what the biological filtration can handle. The reason for the drop in pH is due to the acidic nature of the ammonia. You are right that the bacteria in the biological filtration need time to multiply in sufficient numbers to handle the new load. It is important that you take some measures in order to keep the level of ammonia and nitrites down so they don't place too much stress on the fish.
Ideally, during a cycle such as this, you want to keep the ammonia level down as far as possible. Perform water changes when the ammonia rises above .2ppm. In this situation, you can change as much as 30% of the aquarium's water without placing too much stress on the inhabitants. These water changes should be performed daily in order to keep the toxin levels within reasonable amounts. Aerate the water you are adding and bring it up to the same temperature as the aquarium to reduce any stress that would be caused from a rapid change in water temperature.
Another method for controlling the ammonia in this situation involves using one of the available chemical ammonia removers. Be sure to choose the correct product, because some of these removers are specifically designed for freshwater and saltwater use. Because the bacteria in the biological filtration need to grow in numbers, you do not want to over use these ammonia removers. Use just enough of this chemical media to keep the ammonia at .2ppm. This will reduce the stress on the fish as well as allow the bacteria the needed nutrition to grow in numbers to meet the new loading.
It is important during this time to test the water daily using a quality test kit. Even if you decide to use one of the chemical medias, if the ammonia level increases above .2ppm, perform a water change as directed above.