Help with a sump

[Ximat]

I've recently tackeled an aquarium at a local school. The LFS that was taking care of it abandoned them and left the tank in shambles.

It is a 75g tank drilled with overflow and a 20L sump.

My question is, there are no partitions in the sump, its just a tank. I put some LR rubble in there, but i need to find a way to improve their filtration. Can anyone give me some ideas on how to improve their filtration in this sump with minimal costs?

TIA

 

[electrikat]

Does it already have a skimmer?

 

[Ximat]

Yes, a poor one, a new one is on order.