Hi,
I'm gonna guess you've done everything right with parameters and feedings so it must be bugs. Check under the leaves and look for spots and see if they move when you pass your finger over.
Got some neem oil on a grow shop nearby? On my town the growshop doesn't have anything named neem oil but that's what they use to kill spidermites trips and other tiny insects that eat our plants. I got Spidermite Control by SMC.
That kills almost any bug with no influence on the plant.
All that does is smother them.