Please HELP first grow leaf problem

HI Everybody, This is my first time growing. Just got my mmw card,and became my own caregiver. In the beginning i got 2 blueberry clones gave to me. I built a 4x8 grow room. Picked up a 600 watt ballast, standard hood and a dual spectrum horticulture bulb. I picked up some five gallon buckets to grow in. I bought miracle grow potting soil. I know not the rite shit. But at the time i was told by another grower that it would be good. It turned out OK but a lot of headache trying to figure out how much nutrients to add, because the soil already has nutrients in it. Long story short had to cut every thing in half. I have been using flora nova bloom 1 stage all three in 1, Its easier and I'm getting good results. OK now to the problems, some of my fan leafs are getting yellow spots on them. Only on the tops of the plant, i have the light about 18in from the top of the plant. The topps are running about 78 F. I started getting these stupid gnats as well. do i try and correct the gnat problem? I'm 4 weeks into flower, also what could be causing these problems i have? looked for mites under the leaves, and did not notice any thing other than the spots where going threw the leafs. I would greatly appreciate any help thanks for your time. P.S. humidity 50% average room temp 78 to 80 and cooler at nite I live in MI.

How's your Ph?

Im using RO water. I have no way of testing the soil. What do you recommend?

If you are using RO add calmag

I've grown blueberry 5 times now and your plants look pretty good for four weeks into flower. Are you counting from the the switch to 12/12 or from the appearance of pistols? I've gotten yellowing of fan leaves every time. Kervork is right, you need to use CalMag supplementation if you are using RO water. My advice to you is to wait until 6-8 weeks after you got pistols, not from the time of the switch to 12/12. You will have a much better harvest if you do.

Thanks for the advise guys. I'm not even at 4 weeks yet, I switched to 12 12 Nov 14th I'm expecting a big harvest. I did let it grow for about 2 months tho. I'm here to say if you have the room try it. The plant in the pic is a little over 5 ft tall and over 4ft wide and 4ft long its huge lol. That is why I'm worried about the spots. I'm going to the grow store tomorrow to pick up some cal mag.

Its going ok but you will have leaf drop/yellowing issues if you dont repot, that pot is too small and the plant with its nute/water needs at this stage will suffer moisture stress (pot drying too quicky too often).