I would spray with something like 30% of what you are normally feeding them. and watch. You do not want to stress them if they are unable to process that.When the roots are ill they lose their ability to pump up water,and to clean their own shit off themselves. I would add a little aminos and kelp to help boost their immune system. I try to look at the girls like they have the flu like a person. Sometimes you do not want to eat and thin fluids are better. You might check your roots with a 60x mag to see if the infection is not rising above the water line. Some times though you have to make a stain to see what is attacking you. Even if you are not top feeding them at all I would think the infection is going up the roots. I prefer H peroxide as it cleans up after itself a lot. In my tanks I can drain them and spray peroxide directly on the whole sys. wait 15 min then flush and remove the dead and dying roots. I would then add a 40-60-(60-80) N-P-K with aminos and kelp and run it a day as a dwc.To help get the roots popping again. A lot of times you end up with a bigger root system then before this happened. I have seen a lot of those tubs in dd's section so I am not sure the light is the major culprit. But then a lot of those guys you never hear from again. I have seen some killer grows and heros over night then they fade. I do not if they made a bunch of funtickets and ran or they got hit hard and was too embarrassed to share their problems. This to me is where you separate the chaf,cuz everyone is a killer grower till the shit hits the fan. I wish you the best of luck,and it is far worst to do little then to try something. JK