First i dont think it is heat related, Maybe overwatered, Those ph/moisture/light meters for seven dollars are crap, crap, crap. They are basically only good for moisture. The ph meter on that is so inaccurate it is funny. Red stems can be an indicator of phosphorus deficency. First thing i would do is flush my soil until the runoff or drainage reads a correct ph of between 6.2 and 6.8. Even though i believe you are over watering i would still reccomend getting the soils ph correct first. That perlite should be mixed in with your soil not just in the bottom of the pot, better to use rocks at bottom and mix the perlite in with your soil. also wait till the soil is dry more than an inch down especially if you dont have good aeration the top layer drys out way faster than the rest it is exposed to the light and air. A good method to know when to water is to use light pots, then water feel the pot weight and dont water again till pot is light. I also dont think it is caused by stagnant air.Hope this helps and all grows well

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I have never grown marijuana or intend to. Anything I post is knowledge gained from reading, not personal experience. Any pictures I post were aquired on the web or are fakes.