After adding water conditioner, it is essential to let the tap water sit for at least 30 minutes before putting your fish inside. Adding conditioner to the water immediately can cause stress and harm to the fish if the chlorine, chloramines, or heavy metals in the water have not been adequately removed.
This is because the chlorine and other chemicals in tap water can be harmful to fish, and the conditioner needs time to break them down. If you put your fish in the water too soon, they could be exposed to these harmful chemicals and become sick or even die.
Also, some water conditioners can temporarily reduce the oxygen content in the water. Waiting 30 minutes allows the water to become aerated and reach the proper oxygen level for the fish to breathe.
After 30 minutes, the conditioner will have had enough time to neutralize the harmful chemicals in the water, and it will be safe to add your fish. Ensure to check that all harmful toxins are removed before adding the fish.