Hello, Mark,
I put it in the second left chamber and remove the fine filter.
Funny you should say that.
I initially removed my fine filter as it would clog up REALLY quick causing the water level in the viewing window to drop, causing the ATO to pump more water, it was getting hard to manage.
I wanted to run the fine filter and I read where people suggested "lifting" it up 1/2 an inch or so.
I tried that, but it kind of defeated the purpose of even having it.
So... I thought, there has to be a better way.
What i've done, and it took a few tries to get the proper height, is to replace the white fine filter material with the blue filter stuff that I buy in bulk and use in all my tanks.
This is what it looks like.
linkhere
I cut it and put it in the fine filter holder up to the top bar.
I replace it every couple days, and yes the ATO alarm goes off when I pull it, but it's just for a minute.
I'll throw up a picture of it tonight when I get home.
Here's the picture of what I did to the fine filter.
Not the middle, 2/3's up. Memory is not what i thought it was.