Need help identifying purpose of Reservoir on tank

[buddahi]

This is a new (to me) tank and I have been considering changing the design of my sump to make room for a refugium primarily to start growing the Chaeto. However, the back of my tank has a large sections that does not seem to have a purpose. The overflow is in one corner, the other side contains the return pipe. both are separated by three distinct barriers (with holes cut in them to allow water to flow between them).

A couple questions:

1. Is there a purpose for so much wasted space other than a place for Algae to grow
2. It gets plenty of lights, could I use section 1 or 2 for Chaeto and what if any are the risks?

I hate wasting 10 or so gallons of water on something that doesn't see to add much value.

Thanks for any help you can provide.

 

[weedage66]

I think I had that exact tank several years ago.. If i'm correct on the right side you might have a little green window that let you see the water level in the sump area. And when I got that tank that back area was mostly used for equipment like small air stone skimmer, heater, bio media, and sponges and a power head for a spray bar that ran the length of overflow. what I did was drill the back of it and added a true sump under the tank. You could totally use that area for Chaeto, but its real slim area so maybe use long strips of LED's and grow it the length of the overflow.