Reiterating Fishmanic. The dechlorinators break down/ neutralise chlorine as soon as they come in contact with the chlorine molecules. However, this can take some time if the water is stationary and not circulating.
The best way to dechlorinate water is to fill a bucket with water, add the appropriate amount of dechlorinating agent, and then aerate the mixture (water and dechlorinator) for at least 5 minutes (preferably 30 minutes). This gives the dechlorinator time to come into contact with all the chlorine molecules and the water should be safe to add to the aquarium.
Aerating for 30 minutes also allows the dissolved gasses in the water to stabilise and get back into equilibrium. When water is under pressure (like in water pipes), the dissolved gasses can be forced out. This can leave the new tap water lacking oxygen, carbon dioxide or nitrogen. Aerating the mixture allows the gasses to get back into the water in the correct amounts, making it safer for the fish.
You mention putting bacteria in... What do you mean by that?