Why comets? Comets are probably the least appropriate type of goldfish for a tank of nearly any size.
You'd be best off with some of the "fancy" varieties for that tank. Stick to Ryukin and Shubunkin, some of the Orandas. I'd say you could do 12 to 15 of the fancies in your tank and with that filtration you'd be perfectly fine.
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 3 Planted Tanks: 10g, 20g, and 29g
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