Drew wrote:

I currently have 1 4" inch comet housed in a 10 Gallon tank. I have a
air driven corner filter. (I change the water every week, water quality
has been excellent) I want to add at least 1 other fish, and get a
larger tank - either a 20 or 29 gallon. I have a few questions. 1) Can
I get a fancy goldfish, or should I stick with another comet? 2) What
size tank would be sufficient to house a total of 2 or 3 goldfish? 3)
What would be a recommended filtration setup for the tank? (brand, flow
rate, etc) 4) What is the process I should follow to move from the 10
to the larger tank?
Thank you for your help

For comets, aim for at least 20 gallons per fish. Recommended
filter would be anything that filters 10x your tank per hour, and
anything that has large filter media space. To move from the 10 to a
larger, just move some filter media from your old tank to your
new to seed the tank. Keep with 1 fish until your new tank has
established.

Fancy goldfish sometimes have a hard time competing for food with
torpedo shaped goldfish such as comets and commons, this is not
always the case though. Avoid bubble eyes or telescopes in my opinion,
if you are adding with a comet.