Upper leaves with brown spots

According to a friend and other growers it's fungus, and it appeared because I closed the window of the room where the tent is + locked door + it's the room where I sleep.... So I guess the air was stale and there was not enough ventilation, plus winter has arrived here in the north so it's cold ... the temperatures were between 17 and 18 with the lamp off + the fan had it programmed 15 minutes I turned on per hour at night period of the plants. Yesterday I watered them with calmag and alg-a-mic (4ml/ L water) and activ-vera (5ml/ L water). Cut off the contaminated parts of the leaves and that's it... Wait to see how it evolves before resorting to more intense products. Tonight I am going to spray the leaves with bio-heaven... and we will see how everything evolves...

I bought the calmag today, I plan to give it to them tonight or tomorrow, depending on how dry the soil is, I haven't given them water since Saturday (4 days ago). A friend tells me that it is a fungus, probably because there is no ventilation, I have an extractor fan and a fan and there is a window next to it that is half open, but not too much because it is already getting cold, winter is here.... The extractor fan works all day long and the fan in the night period of the indor in intervals of 15 minutes on per hour, and when the lamp is on the fan works without intervals together with the extractor fan. Here is a picture of my current tent... What do you think?

I'm with AsNoriu here... you've got to figure out WHY you've got this problem before you can solve it. Calcium deficiency is, quite frankly, NEVER an issue of too little calcium being provided to the plants.. because there's sufficient calcium in any reputable nutrient line ... the deficiency can come from several things so just throwing calmag at it is not going to be a guarantee fix... If you've been feeding it too much nitrogen, this will lockout calcium - or if your pH is not in a proper range, this will make calcium simply unavailable to the plant. So figure out what you're doing wrong and you'll be able to fix the problem. If we knew what diary you're referring to, we could take a look at it and help you find the issue... Good luck

Hi there Looks like a calcium deficiency.. Add some calmag if your not already Best of luck 🌱

its calcium lockout, i see some signs of magnesium deficiency too, very bad lightning for photos. reasons could be many, from bad watering regime and ph swing in pot just ph load to pot lower than 6.0 nitrogen toxicity aftermath root bound, one more bad watering regime aftermath even stems are reddish, low temperatures during grow reduces uptake of calcium too ... or already progressed calcium issue too close light with too weak watering, aka underwater can cause same look and state Think and correct.