Sounds like there is water shorting out the digital ballast. I had the same problem on my Rio 180 lighting kit.
In my infinite wisdom I had blown the warranty in the first week of owning the tank as I changed the plug. The plug had become stuck behind a tank full of water and rocks and I had to remove the original one to get the flex out. So, since I was going to have to buy a new Juwel bar I decided I would have a go at fixing the old one.
Juwel heat seal the majority of their light kits so I had to cut the top of the light bar with a saw. When I removed the top part and "too my surprise" the inside of the case was as wet as an otters pocket. water all over the place!! I'm no electrician but I think I found the cause of the problem!!! All the screws and contact points in the bar and on the digital ballast were oxidized.
From viewing the light bar from both inside and out, I am of the opinion that the water must have been gaining access through the four contact points at each end of the fluorescent tubes. The waterproofing were these points go into the actual lighting bar is just a small rubber ring, a not very effective one at that.
This is how I think the water gained access. Physics 101!
When the lights go on, the digital ballast heats up!
This in turn makes the air around it expand. Pressure builds up inside the bar and it is pushed out through the four "not very well sealed" contact points!
Then when the lights go off, the digital ballast cools and the pressure inside the light bar is less than outside and it draws air back in through the four "not very well sealed" contact points!
The only problem is that the air drawn back in is ladened with moisture from the tank. This done continually for a number months will eventually lead to a build up of water in the light bar.
Thus causing the problem with the oxidization of the contact points or may even lead to a short in the digital ballast.
The circuit board looked alright until I turned it over and it became evident that this problem was more than a couple of oxidised contacts. The water must have been accumulating under the ballast, with the result that it was acting as a connection between the live and neutral parts along the board. Much of the plastic coating along the boards pathways has been melted and many of the actual soldering points for the top side resistors had joined together. "I think the ballast had flat lined". I used a phase tester to check how far the circuit was running to and it only got to about the second resistor. I'm no good with a soldering iron and plus I have no idea where I would get the spare part so I think its time to get a new ballast. See an example of the water damage below, after the removal of the melted plastic.
Just to finish off about my episode with the faulty light bar. I remedied the situation by buying an electronic ballast that ran the same number of tubes and wattage as the juwel bar.
This is a picture of the kit!
When I got the kit I stripped off the tube holders, all the wiring for the tubes and power, the protective black casing, and the on/off twitch.
First I had to complete the circuit where I removed the on/off switch and then I added all the original wiring from the light bar into the ballast with the help of my trusty soldering iron. When this was finished, it worked perfectly. Next, I siliconed the crap out of all the possible openings and put the bar back together. It still worked perfectly! and only cost me the bones of €30 to have the ballast delivered to my door.
Hope this is may help someone else as it costs a lot less then then buying a brand new bar. Plus there is no visible sign of the bar ever being opened apart from the bead of silicone on the underside.
Regards