I think that is the place commonly known as FCBZ (Substract one letter from each to get the secret name)
I saw this and was like ".........*stare at screen for 30 seconds*......what....?" and then it clicked. Thanks for frying my brain, Hoppy.
On topic, this is a really nice setup. Seems like this would be perfect for a nano tank the way the light is focused. Great job!