I believe the electronic ballsat in the Rio 240 is for 2 x 54W lamps and the Juwel lamp length is 1200mm.
The nearest lamp you'll get for this off the shelf is 1149mm @ 49W, so moving the lampholders should allow you to use standard length 49W T5HO lamps.
I have the Rio 180 and I've investigated the whole Juwel lighting outrage (Â£25-Â£30 a lamp ) and will be changing my ballast and lamps over to 'off the shelf sizes', when either a lamp or the ballast blows up . It transpires that I can replace the ballast and both lamps for cheaper than I can buy a single Juwel lamp .
In the meantime, I'm a great believer in the motto, 'If it ain't broke, don't fix it', so I'm just going to run it until the first failure. Never know, I might get lucky and get years out of it .
You can have a read about my enquries here viewtopic.php?f=50&t=6750.