How plants look under LED lights?

Im a first time grower with a noobie kind of question about LED lights. So 2 of my indoor soil grown plants started showing early signs of nutrient issues. Im using RO water PH’d to 6.4-6.8 with Cal Mag every time and periodic Zeus Juice additions, and the girls were just 40 days from sprout so I had just given them their first dose of 1/2 strength FF Trio. (They were living off only the FFOF soil nutes before that) Since I had just gave them their first food I thought it was more likely a nutrient deficiency but was not sure. I also just added 1500 ppm CO2 and am running at 85 degrees so I was wondering if that sudden change could be the culprit as well. I had not been checking the runoff for PH and TDS up until this point but I am now. Either way I had just fed them and wanted to wait a day or two before deciding either to flush or feed. If they got better after the feed it was most likely a deficiency. If they got worse it was toxicity or lockout. But each time I looked at them over the next day in the tent they seemed to be getting worse. So to make a long story short yesterday I pulled them out the tent and into the sunshine for a good flushing and I was amazed how different the plants actually looked in the sunlight. They looked so much more healthy and happy. The pale green color and coppering of the large fan leaves almost disappeared and besides some very minor browning of some of the leave edges the plants looked good. If they had looked like this in the tent I would have not been concerned. So different from how they looked under my HLG600 rspec. I ended up flushing them anyway with RO water PH’d to 6.5-6.7 just to be sure. The last few gallons I added Cal Mag and 1/2 dose of FF Trio. Am I crazy or have any of you noticed the same thing. The difference was huge. Sorry I dont have pictures. Should have taken some. Next time I have them in the sun I will.

Also since this is my first grow and first flush…how long does it generally take to notice if the flush corrected the issue. I know that damage to leaves doesn’t regenerate, so do you judge by the new growth only? Watch the new growth for any signs of the issues you were trying to correct with the flush? How long can that take to be noticeable?

Thanks in advance for your opininos.