Yes that’s the vent, and you need to uncap it, it’s hard to say why your siphon doesn't restart on its own without seeing the whole thing. However the U at the top of the tank should have a check valve on the back part of the u, at the top of the u above the part going down into the sump. This check valve lets the out air that builds up in the system and keeps new air from coming in.
here is the problem you will have, if your siphon doesn’t start up by itself, when the power goes out the siphon breaks. When the power returns the pump starts pumping water into the tank but no water leaves the tank, so all the extra water that the tank can’t hold go on the floor. Trust me this that is not fun, you might read through the diy overflow page again, I would hate to see something happen to that awsome tank.
here is the problem you will have, if your siphon doesn’t start up by itself, when the power goes out the siphon breaks. When the power returns the pump starts pumping water into the tank but no water leaves the tank, so all the extra water that the tank can’t hold go on the floor. Trust me this that is not fun, you might read through the diy overflow page again, I would hate to see something happen to that awsome tank.
