Will the BM skimmer fit in the 3rd chamber. I’m getting the IM10 watt uv light today. Takes the 2nd chamber out of the picture
It will. but I believe to be effective you need to have a steady water level.
Which UV?

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I was just wondering where the level should be because I just put water in my IM 15 yesterday and I have the level in the return chamber alittle lower then the second chamber. But I don't know what the right level is. Thanks
There isn't a "right". I do think you should run as much as makes sense to keep the total water volume as high as possible. Running it higher does also reduce the sound of water falling over from 2nd chamber.