Lowering the intensity is more important than lowering the duration. Keep it at 6 for now. When things clear up, plants get healthy and the nutrient regime is fixed you can return it to original levels/duration.
I'll start to double dose and turn up the C02 (going to get 2kg bottle to go with this tank now).
To boost bio mass in this tank a bit, I kept some cuttings of Hygrophilia Polysperama and Rotala Rotundifloria from the other tank and planted those last night. I also cut back the Ludwigia so it is now about an inch above the substrate to try and promote the growing of submersed leaves.