I certainly don't think its necessary to change the media every week. I'd say every two weeks because there's no chance for the flies to reproduce in a week so you will run out. Yes just squeeze the oranges and get juice into the mixture. Sorry I don't know anything about a show in Doncaster. I live further north near the Scottish border so I wouldn't know anything about any Doncaster shows. Thanks for the offer though It's my birthday in November so I may get something from some show though but it will probably be another spider or tarantula. Yes I do have a culture of fruit flys too. It's not for dart frogs though. I use my fruit flys for feeding my baby praying mantids and my baby tarantulas and yes those larvae will be flys one day too.