SUMPM question

[CrewePD210]

I have just bought this 150 with what looks to be a 3 compartment sump. However, it has a very large hole on one end, looks to be about 2" Does anyone know what that is for? Or do I just close it off? Thanks. In the picture it is in the bottom left corner on the smaller tank sitting on the ground.

 

[C_AWOL]

It can be closed off but the hole was likely where the previous owner would connect an external pump.

 

[get-salty]

Looks like an extended smaller sump that was plumbed off of a bigger main sump.