O.K. so you do not run your lights for more than 6 hours because they are weeds. False, they are plants. If you are afraid to leave lights on for more than 8 hours at a time than you yourself can & will grow algae because you are lacking something as well.
If plants have all nec. nutrients as well as carbon than more than 8 hours will not be a problem. I never thought of the plants as weeds. If you enjoy cutting back so much on lights because of fear of algae than maybe you should address your issues with imbalances.
I could run my tank 14 hours photo period a day if I wanted to. Why, because zero nutrient issues are present in my tank because I dose dry ferts & run pressurized carbon. I am not afraid of growing algae, algae will not compete.
If you do not run pressurized carbon than bumping up the excel should help as well as dosing dry ferts to customize your tanks needs.
You should not be afraid to run your tank for more than 8 hours. In nature they run on 12-14 hours a day. If you cannot than you have imbalances. improper lighting spectrum, should be 6500-6700k only, proper carbon, NKPo4, tracse, Fe , Mg , Ca to name a few need to be addressed. Nitrogen cannot crash, same as phosphates . N needs to be above 10ppms & PO4 needs to be above .5ppm at least.