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KrazieKarma I'm going out for a run to digest your info, but a couple of things that come to mind:
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strains:
bubblegum, blueberry, white widow, sativa, purple urkle - different heights, do you have the canopy leveled?
21plants in 3 gallon buckets - 12" diameter buckets? I'd like to see some pics of the plants, that's going to be hard to get in an 8'x3' grow area. Right off my advice would be to make a proper grow cage for your 600 watt light (thread linked below) and put as many plants in it as possible. The remaining plants do with what you want, get another light, plant them out doors, throw them away. With a 600 watt light in a proper 24 square foot grow cage and setup you should be able to grow 6 gallons of cured bud per grow within a couple of grows, if you want to grow in a 56 square foot room with a 600 watt light you'll be lucky to get a gallon.
earth grow, nothing fancy - What kind of medium? Pro Mix? B'Cuzz? Potting Soil?
Water a small amount daily as they droop horribly if I miss a day. - water/feed until water pours out of the bottom of the bucket; use the "lift the bucket" method of determining when the next watering is needed (light); soak again until you get run off. I'll go into why later, very very important. You will probably need to water twice to three times a week. I prefer 5 gallon buckets that I only have to water once a week, you can easily repot to 5 gallon buckets now if you want. It'd give you a chance to rough up your root ball and get rid of the root circling. (post #7 in the thread linked at the bottom of this post, first part, Air Pruning Buckets).
I flushed with low Ph water until the run off came out at 6.0. - up your PH to 6.3 - 6.8 for soilless, 6.5 preferred.
Battling spider mites with AzaMax. - install an air floor. Simple, cheap and will solve problems you don't know that you have yet. The second half of post #7 in the thread linked below.
2-4 feet - plant height
light is -
2 feet from the tallest plant top: Check the temperature of the light with the back of your hand an lower your hand until the heat is comfortable, that's what your distance should be. Probably 18" for a 600 watt, depending on ambient temperature and venting. Perhaps 12" with a light mover. Level your canopy top by putting your short plants on empty buckets, or by low stress training. See thread below.
I recently learned how to make Co2 in the room - Stop. It's useless for you right now. A needless distraction.
the room is in the basement so the door is always open. - Do you sit in the room during lights out? 15-30 minutes with a vaporizer, no light. Let your eyes adjust. Any light leakage? Cab you see your hand in front of your face?
I have an osculating fan that runs constantly and I have an ionizer on constantly. - I would advise building a grow cage (linked thread) and installing a fan to draw the hot air from the top of the cage and an inlet at the bottom (air floor), both for venting heat and for drawing fresh air through the canopy. Reduces mold and pests; increases air across stoma.
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https://www.rollitup.org/newbie-central/360125-new-growers-needed-grow-lab.html#post4545779
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