Ok, so... a couple things.
Take it from someone who has gone back and forth with the pH battle, it's not pH that's your problem. If you're growing in an organic soil, you do not need to pH your water. Soil is much different than hydro. Hydro has no natural pH buffer like soil does. If you pH the water (or don't) your soil will automatically buffer the pH of the water almost immediately upon entrance to the soil.
If you DO decide to pH it (waste of time and resources imho), do NOT use pH down (or up). Those chemicals are made for hydro applications and are not meant to find there way into soil. They can do far more bad than good. So... If you must pH your water, use organic methods to do so. Lemon juice will lower pH organically extremely well. But again, I don't believe pH is the problem.
Temps... you mentioned a heat lamp. What is the room temp and do you have a way to take the temp at the canopy (right at the top of your plant)? While room temps may not appear too hot, it's all dependant upon where the thermometer is reading the temp from. Make sure you're reading it from the canopy of your plants. Unless you're in the 60's without the heat lamp, you don't need it. Low 70's are absolutely fine (maybe not perfect, but not bad at all). I would try to stay mid to high 70's, and out the 80's (only until you've ruled out heat as the problem). 80 isn't a bad temp but if your plant has possibly had some heat stress, it's best to keep it in a happy medium of mid 70's.
Lighting/reflective material..... you're using foil, which to most appears to be a good reflective surface but it is very susceptible to wrinkles and once wrinkled you can't get it back to it's original state. The wrinkles can create hot spots, reflecting light unevenly, and can burn your plants. Mylar (space blankets), can do the same thing if not applied to the surface nice and flat. Flat white surface is your friend. If your growing space can be painted with flat white latex paint, it will be extremely beneficial throughout the duration of your grow. What type of lights are you growing with? By the blueish hue of the photos I'm assuming a high Kelvin cfl? If so, you want them nice and close to your plants (elevate your seedlings to be the same distance from the lights as the top of your juvenile plant). This will prevent stretching (the nodes aka: sets of leaves, shouldn't be as far apart as they are). If the plant is growing upwards rapidly and isn't producing a whole lot of side growth, you're shooting yourself the foot.
Few questions that would help us help you:
What is the npk ratio of your soil (I couldn't read it off the bag)? It should be represented by three numbers ie: 20-20-20, 10.5 - 4 - 10.5, etc.
What is the temp at the canopy of your plants?
What kind of lighting are you using?
What fertilizers or nutes are you growing in if any?
What has your watering regimen looked like over the last couple of weeks (volume and frequency)?
Hope some of this helps. I (and others) can give better assistance when the above questions are answered.
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