Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
It is normal for some water.
Trapped in the water may be any of various gases, or it may be lacking in gases. Either way are not unknown.
When your water (lacking gases) is exposed to air it starts absorbing CO2, and the pH drops.
You can make this happen faster by adding a bubbler or small fountain pump to your storage container. Put the pump at the bottom, aiming vertically. This water flow pattern is called a flume, and is the most efficient way to circulate water so it all gets exposed to the air. This allows gas exchange to happen quicker, so the water is ready for a water change sooner.