Have you guessed what it is yet?

[atoll]

Some sort of mechanical/ biological in sump reactor?

Getting closer

 

[atoll]

A cap to the sump so gorilla crabs and such wont get out????? Maybe to stop a fish from getting into the sump????

Nope

 

[atoll]

its a prefilter to keep your return pump clean and block invasive crap form getting back into tank

Nope

 

[Reef Jedi]

Bio ball chamber?

 

[atoll]

Bio ball chamber?

Nope.