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Please do NOT trust the "nice people at Petco" as they have a tendency to lie about a fish's needs to make a sale. Common goldfish can get as big as 24" and common plecos larger than that, and should only really be housed in very large aquariums (far larger than a "beginner" would likely have) or a pond. Do research on any live animals you may want to purchase online before even thinking about buying them at the store. I would also recommend checking out any local fish store there might be in your area. Family owned, small town fish stores provide healthier fish and better advice than a large "box store" like Petco, PetSmart, or WalMart can offer.