Set up the 20, move your old filter to it, and what ever decor you liked from the 10 and put the fish in. There is enough bacteria to support your fish in the filter and decor. I have done this type of switches many times and have never lost a fish.
As for moving the fish. Use a 5 gallon bucket and fill it with some of the water from the 10. I have kept fish in buckets like this unfiltered over night and had no problems. Switching tanks shouldn't take more than a couple of hours. Now if you were doing something bigger, I would look for more stable long term space for them.
As for moving the fish. Use a 5 gallon bucket and fill it with some of the water from the 10. I have kept fish in buckets like this unfiltered over night and had no problems. Switching tanks shouldn't take more than a couple of hours. Now if you were doing something bigger, I would look for more stable long term space for them.
