06-09-2012, 12:54 PM
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There is no nitrifying bacteria in the water, but there are some other bad things, so moving water is questionable, as I will explain below.
Do you intend to use the same substrate? Presumably more will be needed in the 40g than what is in the 20g.
If you do use the substrate, and any decor items now in the 20g (rock, wood, etc), do not wash any of these as this might remove/kill the bacteria. When you get the substrate in the new tank, half fill it with fresh water using a dechlorinator (water conditioner) to avoid killing the bacteria in the substrate. Then move over the decor and any plants.
At this point, if the GH and pH of the tap is close to the existing tank, fill the new tank (use dechlorinator again). Drain half or more of the water out of the 20g, and refill with fresh (like a water change). This can be repeated. Then net over the fish.