I wouldn't even put it in if they are common goldfish. They are quite comfortable with that temp. Lower water temp means higher oxygen content, and less bacteria. Goldfish have higher oxygen requirements than most tropicals. That is a small tank for goldfish, and they will soon outgrow it. Daily temperature fluctuations will not bother them, as evidenced by fish kept in ponds, where there is a daily fluctuation. All my livebearer tanks are unheated. Temps range from 66F in summer to 71F in winter. Livebearers live longer at lower temps.