A mate of mine has an old cement laundry sink that is the ideal tank for mullet etc.
I have kept them alive for three weeks in a large esky with flyscreen over the top and an aerator. You have to regularly change the water and I have found that if you have poddies for a period you cant feed them bread as they seem to bloat up and die.
You can drag a bit of weed out the flats to feed them on. They seem to eat the weed that is black, flat, about 3-5mm wide with serated edges.