Fish-killer

A few weeks back we went to a BBQ at a friend's house, where they had recently had fish that all died. Somehow I was rail-roaded into taking their fish tank and stuff home while their neighbour gave us 4 goldfish from their apparently abundant supply. It took 4 days for the first one to die, followed by the second last Friday and the third on Saturday. I don't know for sure what's killing them but it could be me. It could also be the fact that our house is just too damn hot for keeping even tropical fish alive, there is a thermometer inside the tank which sits consistently up around the top of the red zone (which I believe is the ideal temperature for this type of fish) and sometimes goes over the top, despite me trying to keep the house no warmer than around 29C. Or it could just be luck - I also can't keep plants alive. Maybe I'm lucky the kids have become somewhat self-sufficient and can feed themselves at need.