We just got a 20 gallon tank a couple weeks ago for our daughter's 5th birthday. I haven't had a fish since my goldfish that I had a few years ago (NEVER underestimate the ability of a 2 year old.... they CAN catch a little goldfish in a 10 gallon tank....)
I understand you wanting to give him more room. My little fish started out as a college dorm floor "fish survivor" contest in a little fish bowl. I ended up being the main one in my room to take care of it so it became mine. I felt bad & upgraded him to a 1 gallon tank. Then when I got an apartment I gave him a 10 gallon tank! lol
That goldfish was the only fish experience I had. Now I have mickey mouse platys, red wag platys, neon tetras, black mollies, a silver molly, an algae eater, and now a couple catfish (which I have NO idea which specific kind they are - so that may have been a mistake!! LOL). Entirely too much fish for our tank, thanks to my husband & whoever he talked to at Petsmart..... lol
Petsmart is nice though. They have a lot of fish, and they have them color-coded to show which fish can live together in a tank.
I still don't know what I'm doing though :) Oh and one of our mickey mouse platys was (is?) pregnant, so now we have at least 3 baby fishies! Not sure if there were more at one point, but we finally saw that there are at least 3. They are so tiny! Good luck to them.... since I don't know what I'm doing! haha! I just hope the catfish don't eat them!!