Dont buy feeders they are not bred to live long. They are so inbred they wont last 3 months. Well most. Go out and get yourself some sarahasa Goldfish. Little more expensive but worth every penny. Also when you buy remeber to choose the fish you like the most from viewing at the top b/c you will never see the side of the fsih
As for temp fluctuations keep the barrel in the sun for the morning and in the shade for the hot part of the day.
Also Goldfish dont like alot of light so some floating plants like water lily's will help your fish out alot. This was our experience with our pond. Otherwise the fish just stay at the bottom or in the shade.