Hi,

I am a total novice at pond keeping, but my filter has a uv incorporated in it, It is a Blagdon minipond and the 9w uv is part of the water inlet arrangement and as such is before the foam/ceramic filters.

I have only just connected the uv filter and on opening the lid there is a definite visible light to be seen, so in answer to your question of visibility the answer is, yes, the light is visible. Mine has only been on for a few hours so far and i am hoping the water will start to clear soon.

I will monitor it for a few days and update you when (hopefully) my water starts to clear.

Hope this is of some use to you.