Fiji Advanced Sump Question

[JNalley]

Considering tearing out the AIO on my aquarium to give me more room in display, I will be drilling out and adding a sump. The Fiji Sumps seem well built and perfect for what I need. That being said, I was looking at the pictures of the Advanced 24 and cannot figure out what one of the holders is from. It's in the return chamber. I have circled all the "holders" the ones in blue I already know what they are for, but the one circled in red I have no idea, can someone help me out?

 

[V A R I A N T]

That is for a mechanical float valve for your ATO

 

[JNalley]

That is for a mechanical float valve for your ATO

Hah, thanks, I'm an idiot, I should have figured that out. I was scratching my head trying to figure out what it could be for.

 

[LoganBailey22]

That is for a mechanical float valve for your ATO

Will any mechanical float valve work?