Comet Goldfish?

No. They get huge.
 
i agree with the above, ps i like the snail porn in your avatar haha
 
If you get em when they are 2" you might be able to keep them in a 20g for about 2 months, then you would have to upgrade. If you had no plans of upgrading then dont bother getting one, they grow pretty fast.

Got mine when they were about 2-3" in around a years time they are 4x that.
 
Fantails get too big.

I think you should stick to tropicals, or get a bigger tank, and do what's best for the fishies health =)
 
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