New 20 gallon overflow?

[Bennutri]

Should I leave the block off and keep it w the canister filter or take off the glass silicone block and add the overflow?

 

[GoVols]

If your running to a sump, then you want the surface skimmer "block" screwed onto the drilled hole.

Looks... ;Wideyed like a drill hole is on your first shot???

If not, the plug the hole with the right sized bulk head with plug, and fit a protein skimmer inside the back chamber.

Welcome to Reef 2 Reef!!