My first Juwel unit lasted 7 years and that was bought second hand. Current T8 unit is 4 years old and is about to be replaced this week with T5 hi-light unit.Also juwel lights dont have a good track record, their ballasts are known for being short lived
Most Juwel lighting failures, according to local shop is people dropping the unit in their tank and water getting in, especially in older units where the seals fail at the lamp connectors and the fact that people fail to seal/replace the lamp gaskets.
If I had my way, and could get the Juwel units open (and sealed again) I would be to replace the Juwel ballasts with a dual dimmable version (about £50) so the tubes could be dimmed. There was a dimmable ballast I found based on 1-10V control and a switched one where you apply power for 0.6s, turn off and when back on cycles up and down brightness until you cut power again and it remembers that brightness.