overflow sump HELP
 

so i plumbed it for the return and the larger 2" overflow i just figured the 1" bulkhead was so the tank could be drained completely.... till i put my finger in the hole and realized theres no cap/or threading....

so stupid question...... whats it for??
should i just put a 1" tube through it as a double overflow?

the sump is set up so it goes... from tank - skimmer - bubble trap- mechanical- live rock - return pump-tank

i understand that you have 2 holes in your over flow box.(i don't see why one is smaller than the other)
if that is the case i would use the smaller one as an emergency drain to the sump in a Herby setup.

The smaller of the two holes is usually for the return line from your pump back into the tank. You, of course, do not have to use it for the return line and an emergency overflow is also a good use of it.