Thank you all for your many replies!
My solution tank capacity is 45-50lt, each day 5 are consumed by 4 plants, which are keep growing towards the lights, probably because of the heat. Summer is a difficult season to grow indoor, but in the night I keep the air circulating through the window, blowing out hot air with a spare extractor. I added a fan for the intake flow, blowing towards the plants.
pH values are usually increasing each day by 0.3-0.4, so i keep them in the 5.8-6.1 range.
I always check pH and EC values before and after topping, which worked fine in the spring, but with raising temperatures raised also evaporation, if not from the plants, from the tank itself. Everyday I have to lower EC by 0.3mS. I will try with a much lower EC and see what happens in the next few days.
My lights are currently as high as they can be and yet a few top buds are growing towards them, some are already touching. I thought it could even be too much light, but the top leaves are the healthiest, the lower leaves instead are scorched.
I don't foliar spray, the only one I did it was a few days ago on the lower leaves, with pyrethrum and mineral oil, which is organic and decays fast.
I've just brought down EC to 1320uS, pH 5.9, I hope to see some changes.