My water pump failed and it appears that the heat from the UV light warped the case of the ballast,
thus causing the water to leak into the UV light tube and short it out.
I have tried using sealant to keep the water out but no luck.
Does anyone know where I can buy a replacement outer casing?
Oase only sells a completely new ballast and that's expensive!

Sorry I am late (Don't come here often) Sorry to say, but if it is made by Oase, the only place you can get a "spare part" is Oase. As they do not sell it as a spare part, unless you can get a whole one (donor) on the likes of ebay there is nothing you can do. There is "no money in it" for anyone else to make it as a spare.