If the pH is coming out too low, you should be putting it in at the max recommended for your medium until it starts coming out in the range. So if you're in soil, you want 6-7 pH. If you're putting it in at 6, and it's coming out at 5, that means the pH in the soil is too low. So you should be giving water at 7 pH every time you water until it starts coming out in the 6-7 range. I've found that helps correct the deficiency without needing to flush. I hope that helps!
The other thing is to consider the LED because sometimes you can light-burn your plant with one being too close, especially seedlings. What distance does the manufacturer recommend the light to be from the plant?
Overall your seedling actually looks really good!
