Newbie needing help with yellow spot found on leaf.

Hi all, I am a newbie trying my first grow here and starting to see some minor problems, any help would be appreciated.
I have a northern light plant here that i have under a 250w hps light 24/0. The light is 18 inches from the plant.
Growing in root riot from seed. I have been spraying it with ph5.3 water and have been doing so for 9 days now.
Today i have found a yellow spot on one of my leafs. I have been looking through the forum for answers and i am wondering if i should start giving it nutes now ?
I will be using canna vega A/B to veg then canna flores to bloom with DWC in 5 gallon buckets.

Thanks in advance.

Spraying your plant with 5.3ph is a little too acidic, also it's not needed to spray your plant, if you intend to go dwc your ph should be at least 5.8, if your seedling is in just pebbles I would give it a very week feeding of nutes, your seedling may be hungry, hence the beginning of yellow spots, that's what I do with no problems.