It looks very nice and I am sure Aquarius will be very happy in there. You will have to let us know how he likes to have the extra room to swim around in. There is a nice balance of swim room and decoration and I love your plant. He will love to sleep in it. Thank you for thinking of him. He will appreciate not having to swim so far at night when he needs to go to the surface to breathe. Are you doing a fishless cycle? The fastest fishless cycle is the Bio-Spira where you can put him in immediately. The next fastest is the one with pure ammonia which can be done in a few days. The cycle done with fish flakes takes a few 4-5 weeks as I understand it, but I personally have never used that method. Maybe Gwenz could tell you as I believe that is the method she uses. I used the Bio=spira method and one time the Ammonia method. And one time a combination of the Bio-Spira and Ammonia method and it worked beautifully. (the fish was coming by mail and I wanted the tank completely ready 100%).
Yes I use the fish flake method to cycle my tanks. It takes about 4-5 weeks. It does take a while but I'd rather wait this time thatn put a fish in an uncycled tank. All you ahve to do is put a small pinch of flakes into the tank once a day. The flake will float around for a while and then sink to the bottom and they disintegrate and that helps the tank cycle. I'm not sure how that happens exactly though. My Q tank took 5 weeks to cycle, and I'm currently cycling another tank using this method.
Here is some more information about cycling with fish flakes (Look at Option 1 under "Starting the Cycle Fishless") :
Sorry I am not thinking well today. I missed that you already had a cycled filter. No you don't have to take your whole filter out of the 20 gallon. Dino starts more tanks than I ever have and he tells how to start a new filter in the easiest way I have ever heard right here so just follow his instructions and all will be well. It sounds like all you have to do is take the filter for the new tank and run it in the old tank for a week and then put it in the new tank and just don't overload it with fish for a while. Just putting a few Mollies (2 or 3) in would be great. Be sure and read what he has to say.
Dino's tanks are used to house fish for resale and are huge so I am sure the tank size is not a factor, but if you are concerned I think you could PM him and he would let you know. I think the idea is just to have the filter that you have running as a second filter be big enough for the new tank and it will take care of seeding itself. You actually have both filters running at the same time in the cycled tank.