During the first two to three weeks after the setup when the filter system (biological filtration) is not yet fully functioning, the aquarium water may contain ammonia and other substance which can harm the fish.
You would feel like adding fish to the aquarium immediately after the planting is done and the equipment have been installed. However, you should swallow the temptation and wait. One of the failures frequently observed among beginner aquarists is adding fish to a newly setup aquarium and ending up seeing it dead. During the first two to three weeks after the setup when the filter system (biological filtration) is not yet fully functioning, the aquarium water may contain ammonia and other substance which can harm the fish. It is advised to add fish to the aquarium only after the aquatic plants firmly take roots and grow lush and healthy. Conduct a water quality check using Pack Checker to make sure that the environment within the aquarium is good for fish.