Any T8 tube of the right length will fit, you don't have to buy the Juwel parts. I personally used to run the Hagen range of tubes for T8, and they are good reliable tubes, available in many "colours". Some tubes enhance plant growth, others fish colours, others try to simulate daylight. The tube of choice realy depends on the set-up 
/www.juwel-aquarium.de/en/monolux31.htm?cat=40
The light has been in the tank just over a yrear so it needs replacing! the plants struggle as it is with only having one light tube so it will be nice to get a new brighter one, that hasnt lost light!