So you've made your decision and bought some folks from many aquarium fish species that are available in your local store
. I hope you have choosen your crew wisely so they won't eat one another:)
Now you have your fish and want to place them inside your tank, you can't just drop them in. Water in your tank and the water in bag that you brought from store propably have different temperature, might have slightly diffirent chemical parameters. Fish don't like that sudden change, and might became very nervous which can lead to their sickness or even death. So you must acclimate them slowly.
Here are two ways of doing it:
First one is for fish that are more resistant to stress, basicly you must prepare a little bucket and a cup. When you bring your bag with fish just gently trasnfer them (with water!) into the bucket, if the water level is not high enough you might want to tilt the bucket( carefully) a little bit, use a wall as support.
Then every 10-15 minutes pour some water into the bucket, beforehand remove equal amount from the bucket. Continue exchanging small portions for about one hour.
Second one is for more delicate fish it is called the dropping method. You will need a thin tube with valve and of course a bucket. Then you pour water into the bucket (with your fish inside) using your tube and manipulating the valve so the flow of water is something like few drops per second.
Next you remove some water from the bucket, you will finish the whole process when you exchange about 100% the capacity of your bucket.