Your pH is good at 5.8.
pH rises or falls in a hydroponic res as a result of exchange of chemicals between the nutrient solution and the plant. In veg, pH will tend to rise, in flower it will tend to drop. This is completely normal and is a reflection of which chemicals are being taken up more quickly at the different stages of growth.
In hydro pH will drop if there is biological matter in the res, ie root rot. The way to stave off root rot is to keep res temps < 70° and/or use chemicals ("running sterile") or beneficial bacteria ("bennies"). Per below, I keep my res at 68° and use Hydrogaurd. If you find pH dropping in veg, the culprit may be root rot.
PPM of 800 is staggeringly high for a small plant. My approach has been to use about 300/500ppm for seedling, then jump to 500/500 in very and maxing out at 800/500ppm in flower.
What are the temperature and RH readings?
The goal of adding nutrients is to allow the plant to have a sufficient amount of chemicals to allow it to reach its genetic potential. If you need to grow a seedling at 800/500 ppm, that's a sign that something is wrong ini your grow environment. The goal is to provide the minimum amount of nutrients needed to hit the sufficiency range. Once you increase EC past sufficiency, you get to toxicity.
Measuring by volume is OK to mix nutrients but it's not as accurate as using a scale. Think back to chemistry class - did you measure in tablespoons or grams? If you're going to measure by volume (a tablespoon, teaspoon, etc.) make sure to run a straightedge across the top of the spoon. That will help get your measures more accurate.
The standard practice to determine the nutrient strength in hydro is to use an EC meter. PPM is not a measurement. PPM is a calculation that's based on EC and, since it can be converted using three different sets of values, it's easy to miscommunicate using PPM, EC is the standard because it's an unambiguous. Having said that, I prefer to use PPM using the 500 scale which is why I add "/500". (My reason for using PPM is that my res holds 26 gallons of nutes and it takes many days to detect a drop of 0.1 EC.)
I would never "keep adding nutrients until PPM stops falling". We know the approximate EC values of where cannabis plants hit the sufficiency range so there's no valid basis to expirement. In addition, the chemical composition of nutrient solution changes…hourly so EC may stay that same while the nutrient solution has a very different composition. It's hard to f it up by giving a seedling 300/500ppm±.
EC is a measure of much electricity the nutrient solution can conduct. When a solution is fresh, it's an acceptable proxy for nutrient strength. Within a few hours, the plant may have absorbed a good percentage of the chemicals in the nutrient solution. EC may not change but that doesn't mean that nutrients are not being taken up - it just means that the EC hasn't changed and that's because the different chemicals in the nutrient solution impact EC differently.
The photo below shows a plant that is 13 days from germination. Note the LCD display in the background. That's a Bluelab monitor showing PPM (500 scale), res temperature, and pH.
The roots for your plant are markedly smaller than in the photo. At 800/500, that's a huge nutrient load so that will impact growth but if your temperatures + RH are way off, that will impact growth as will light levels.
What light are you using, what's the hang height, dimmer setting, and light schedule?
Cannabis does not need darkness to do its "dark cycle processing". I have run my plants 24/0 until flower but have switched to 20/4 for the past few grows. If you run 18/6, that means that your plant is only photosynthesizing for 18 hours. My preference is 20/4 which is about 10% more time with lights on than 18/6.
This plant has been sitting in 26 gallons of nutes for two weeks and the roots are white as the proverbial FBI lunchroom. New roots should be white but, as they get older, they will get stained by the nutrient solution and will gradually turn brown. Roots should have a "clean dirt" smell. If they have an off odor or as slimy, that's root rot.
I keep my water temp at 68° and use Hydroguard and have never had an issue with root rot. Some growers "run sterile", meaning that they add chemicals to kill bacteria I just chose to "use bennies" ("beneficial bacteria") even though I'm a "better living through modern chemistry" guy.
The factor that hasn't been discussed is how much light you're giving your plants. Don't go overboard with too much light but very few growers do go overboard. It's quite the opposite - growers lose more of the crop by not giving plants enough light than they do by giving their plants too much light.
This image might help you set up your grow. It looks like the nutrient strength issue is resolve but there are a handful of other parts of the grow environment that you might want to address.
pH rises or falls in a hydroponic res as a result of exchange of chemicals between the nutrient solution and the plant. In veg, pH will tend to rise, in flower it will tend to drop. This is completely normal and is a reflection of which chemicals are being taken up more quickly at the different stages of growth.
In hydro pH will drop if there is biological matter in the res, ie root rot. The way to stave off root rot is to keep res temps < 70° and/or use chemicals ("running sterile") or beneficial bacteria ("bennies"). Per below, I keep my res at 68° and use Hydrogaurd. If you find pH dropping in veg, the culprit may be root rot.
PPM of 800 is staggeringly high for a small plant. My approach has been to use about 300/500ppm for seedling, then jump to 500/500 in very and maxing out at 800/500ppm in flower.
What are the temperature and RH readings?
The goal of adding nutrients is to allow the plant to have a sufficient amount of chemicals to allow it to reach its genetic potential. If you need to grow a seedling at 800/500 ppm, that's a sign that something is wrong ini your grow environment. The goal is to provide the minimum amount of nutrients needed to hit the sufficiency range. Once you increase EC past sufficiency, you get to toxicity.
Measuring by volume is OK to mix nutrients but it's not as accurate as using a scale. Think back to chemistry class - did you measure in tablespoons or grams? If you're going to measure by volume (a tablespoon, teaspoon, etc.) make sure to run a straightedge across the top of the spoon. That will help get your measures more accurate.
The standard practice to determine the nutrient strength in hydro is to use an EC meter. PPM is not a measurement. PPM is a calculation that's based on EC and, since it can be converted using three different sets of values, it's easy to miscommunicate using PPM, EC is the standard because it's an unambiguous. Having said that, I prefer to use PPM using the 500 scale which is why I add "/500". (My reason for using PPM is that my res holds 26 gallons of nutes and it takes many days to detect a drop of 0.1 EC.)
I would never "keep adding nutrients until PPM stops falling". We know the approximate EC values of where cannabis plants hit the sufficiency range so there's no valid basis to expirement. In addition, the chemical composition of nutrient solution changes…hourly so EC may stay that same while the nutrient solution has a very different composition. It's hard to f it up by giving a seedling 300/500ppm±.
EC is a measure of much electricity the nutrient solution can conduct. When a solution is fresh, it's an acceptable proxy for nutrient strength. Within a few hours, the plant may have absorbed a good percentage of the chemicals in the nutrient solution. EC may not change but that doesn't mean that nutrients are not being taken up - it just means that the EC hasn't changed and that's because the different chemicals in the nutrient solution impact EC differently.
The photo below shows a plant that is 13 days from germination. Note the LCD display in the background. That's a Bluelab monitor showing PPM (500 scale), res temperature, and pH.
The roots for your plant are markedly smaller than in the photo. At 800/500, that's a huge nutrient load so that will impact growth but if your temperatures + RH are way off, that will impact growth as will light levels.
What light are you using, what's the hang height, dimmer setting, and light schedule?
Cannabis does not need darkness to do its "dark cycle processing". I have run my plants 24/0 until flower but have switched to 20/4 for the past few grows. If you run 18/6, that means that your plant is only photosynthesizing for 18 hours. My preference is 20/4 which is about 10% more time with lights on than 18/6.
This plant has been sitting in 26 gallons of nutes for two weeks and the roots are white as the proverbial FBI lunchroom. New roots should be white but, as they get older, they will get stained by the nutrient solution and will gradually turn brown. Roots should have a "clean dirt" smell. If they have an off odor or as slimy, that's root rot.
I keep my water temp at 68° and use Hydroguard and have never had an issue with root rot. Some growers "run sterile", meaning that they add chemicals to kill bacteria I just chose to "use bennies" ("beneficial bacteria") even though I'm a "better living through modern chemistry" guy.
The factor that hasn't been discussed is how much light you're giving your plants. Don't go overboard with too much light but very few growers do go overboard. It's quite the opposite - growers lose more of the crop by not giving plants enough light than they do by giving their plants too much light.
This image might help you set up your grow. It looks like the nutrient strength issue is resolve but there are a handful of other parts of the grow environment that you might want to address.