Hydrogen sulphide messes the water up for around 14 days. Add a scream of 1 cm sized air bubbles in the rear corners of the system for that time. Or leave in place, it's good to have air like that. Only cm sized bubbles. You may adjust so a constant stream. Produce them from open ended airline. Not from an airstone. Microbubbles (less than a cm) are to be discouraged. If you get lengths of 6 mm OD clear acrylic tube from eBay, then you can run the air there aesthetically. Don't add to the sump, the depth is insufficient and dragging air into return pumps can cause gas bubble disease. That's another story for another day. Good luck.
If you remove the sand bed and nitrate becomes too abundant too, PM me, I've got a cheap but very effective solution for that.