If you are in soil, pHing isn't as important as soil has natural buffers. Please brush back your plants and take a photo of the media/soil. If it is soil then there are certain things that it could be. Coco would be different. Doesn't mean that it isn't affecting it but it looks more like the plant isn't getting enough food. Going off your post saying you've only watered them twice with distilled water I would say your first issue is nutrition. Rather than buying a one item supplement, like iron, to try and address individual issues I would first start with both Grow and Bloom base. They should both have everything the plant needs for its basic nutritional needs. I would use synthetic nutrients for this run to be able to get the plants what they need immediately. The soil biology you have your plant's in isn't enough for them to be able to draw the nutrition they need organically hence the issues you're seeing.
I would address the feeding part first. As a recommendation I would recommend a product from
General Hydroponics called FloraNova. It is an all in one fertilizer that is a version of their famous Trio line. I use it and it works very well. I also use GH's
CaliMagic which is their Cal/Mag product. Those two products alone should be able to get you to harvest. I only use half of what the GH recommends and have had stellar results.
Look into buying a reliable way to measure pH and PPM/EC. FYI PPM means Parts Per Million and EC means Electrical Conductivity. Two different ways to measure how many nutrient ions are in the water solution. I use PPM to measure the nutrients I give my plants but many like to use EC also. Your choice. If you don't plan on growing after you harvest these plants I would look into pH strips to be able to get a ballpark of where your pH is. If you do plan on doing another grow I'd recommend pH and PPM/EC pens from either
BlueLab or Apera.
BlueLab breaks theirs up into two different pens whereas Apera combines it all into one. The Apera pen also has a replaceable probe. I've been growing for many years so
BlueLab was the only real reliable pen out there at the time and I've used it for decades without fault. I haven't used Apera but I've read rave reviews from trusted sources. Again, your call.
As far as your source water, get on the internet and see if you can bring up some sort of water report from your local municipal water district. They usually put out annual reports of what is in the water so you can decide whether you want to continue using distilled water or just go with tap. My opinion is that most muni water is safe to use other than the chlorine or chloramines they use to clean the water. If you see a high concentration in your water report let the water sit for a couple days to allow the chlorine to evaporate. If your muni uses chloramines then you'll have to wait another day or two as chloramines don't evaporate as fast as chlorine does. This is if the concentration reported is high. A little in your water is no big deal and you can use it straight from the tap. In my area the chloramine levels are low so I don't bother letting it evaporate. There is also a decently high concentration of calcium and magnesium in my muni water so I don't bother with RO or distilled water. It also cuts down on the frequency of my having to buy more Cal/Mag. I use it in addition to what is in the water to make sure the plants are getting the required amounts.
Once your plants get healthier I would raise the temperature to 77°/25C if you have that capability. Do you have a way to measure the specs of your environment? A Hygrometer is essential in being able to monitor the environment your plants are growing in. Humidity between 50% - 55%. 60% is ok but your chances of mold increase. I've grown in 60% without issue but ideally you want to keep it around 50% - 55%. Once they go into flower you can drop the humidity to around 45% to keep bud rot at bay.
When you water you want to water slowly to make sure the ENTIRE media gets wet. Not sure what type of container the plants are in but I'm going to assume they're in cloth pots. When you water, if water starts coming out the side of the pot you are watering too fast. Stop and let the water soak in then resume watering until you see water coming out the bottom of the pot. Cannabis likes a moist to wettish soil so letting the soil dry back is important for the plants growth. Too much water too frequently will result in overwatering issues which seems to be the number one issue most new growers run into. Again, water THOROUGHLY but not frequently. A good way to determine when they need to be watered is the light pot method. right before you water your plants pick them up and feel how light they are. After you water them feel how heavy they are. Now you can determine at what stage the media is in regard to watering needs.
