I'd add an Ocean clear canister post filter, maybe a UV.

You can also use those sock type filter bags if you set the sump up correctly.
These come in 1-5-20-50-100mic sizing.

I'd also add some good size sponge filter pads between the bio and the pump.
This will add a little course mechanical but works well.


The other thing that can be done, this is the cheapest solution, try adding some purigen(seachem) and/or activate carbon etc. and see.

Regards,
Tom Barr