While Franklin has good arguments for the kind of pet he wants, the "best reason of all" is that Franklin loves goldfish. Is Franklin's love of goldfish itself a reason, or is it the result of his reasons? This, at least, confuses facts and conclusions, but is probably also confusing feelings with reason. Should children experience this confusion as a normal approach to decision making?
The story's conclusion also detracts from the theme of Franklin's responsible pet ownership. We are told (and shown) that he watches Goldie, and blows kisses to her, but we do not see him care for her.