New Pond

I have a small 3x3ft 275Lt pond in the corner of my garden half sunk/half raised, which is home to three 7in goldfish that have been in there since the end of September 2014 (and multiple frogs). I've since learned its way too small so I have purchased an above ground 1000Lt 6ft x 3ft x 2ft one which I intend to put on my patio. It will get full sun from about 12.30pm till 3.30pm. I worry that the water will get too warm and kill the fish though. Any ideas on how to make sure it doesn't get too hot? Its going to be sitting on 2x3 slabs. My small pond is in a shady corner and the max temperature it got to last year was low 20's C.