When you are looking at the wall of fish tank chemicals at the pet store you should always take the time to read the label of the tap water conditioner that you choose - they are not all created equal. The bottle should at least state that the product can:
- Remove Ammonia, Chloramine, and Chlorine
- Detoxify Nitrite/Nitrate
- Remove heavy metals
While letting the bucket of water sit out for a night or two will get rid of the Chlorine, aging it will not get rid of the Ammonia, Chloramine, or any of the heavy metals. These are all very toxic to your fish, and thus very important to remove. You will want to add the full amount of tap water conditioner to your tank every time you do a water change. For example, if your tank is 55 gallons then add the amount of conditioner specified for 55 gallons - not just the amount of water that you are adding back into the fish tank.