All mix is too hot for seedlings : )
Light mix is better
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Hello @HierophantGreen welcome to Og
Im not familiar with bio bizz or soil but assuming thatās OK then thereās no reason your plan wonāt work with some fine tuning, I flip seedlings to 12-12 but Iām using coco fibre and mineral salts.
Fwiw starting seeds or seedlings @12-12 doesnāt guarantee small compact light yielding plants, some of best yielding plants Iāve had were grown in that way.
Theyāre not one off examples but donāt want to post a ton of photos.
OK one more 
If yeild isnāt your aim maybe 4 plants in your space would fit better.
Good luck
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What beautiful beasts you have grown, congratulations.
Are you telling me that you grew the plants seen in your photos with the 12 hour light method from seed?
in the rollit up forum a guy told me that for a 400watt hps 3 plants are the optimal number, same yeld as 6 small, but using the 12 hour light from seed method Iām not sure that 3 plants in 12L pots guarantees maximum flowering since Iāll skip the vegetative phase.
I preferred a small SOG with 6 low plants with the central branch full of flowers, but Iām not sure about the size of the pots.
The seeds i have are RoyalMoby 35% indica 65% sativa from royalqueen seeds.
Could 5 liters can keep them low and have the central branch swollen with flowers work for 6 plants under a 400watt hp and 12 hours from seed? I aim for the best yeld i can get whit 12 h from seeds and 400hps light, so 6 small or 3 medium? 
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A plant does whatever it wants to do. The only way to limit the height is to top and scrog a plant as it grows. You can bend and flop them to keep the height to a minimum but that stresses the plant out and usually does the opposite. A 120x120x200 under a 400w light can yield a lot if done right. Youāve got 2m to play with so why limit their height??
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no ventilation?
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Here you can find more info ā¦ 
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@HierophantGreen
Almost 12-12 from seed, I give them 5/10 days veg @24hrs then flip them, fwiw at 12-12 thereās still roughly 4wks vegging period.
Lots of people start on 12-12 on day #1.
The ones Iām growing atm are 7/14 days due to different seed lots but generally Iād give them 5/7 days of 24hrs
The day I flipped.
I used 5ltr pots in the pics but again Iām not familiar with growing in soil?
I use coco fibre and mineral salts, soil requires a different approach which i donāt have any experience with.
My method of training is basic i only tie and/or net them.
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If you want short plants keep your day and night temps as similar as possible. Iāve never used HPS lamps but LED lamps with adequate power help to reduce plant height also. Supercropping helps to limit height as does LST.
Welcome to OG mate.
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If your light is dimmable, IMO the trick to keeping the main stem low is keeping the light close to the plant. In the case of HID lighting, the radiative (?) heat is intense so dimming the bulb helps. Having a CMH bulb for the first 2-4 weeks would be even better but thatās an added expense 
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Link to @PsillyRabbit 12-12 grow.
an Oscillating fan(s) (even/especially for seedlings) helps make strong stems that will not stretch more than needed if your light is in the appropriate location.
im gonna double back and say - no ventilation ???
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When are you going to install the ventilation system?
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As others have mentioned ventilation is vital, itās a non starter without ventilation imo.
Youāll get poor growth, rot and mould.
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Personally, I always grow 12/12 Photoperiod.
Air circulation is KEY, I use AC infinity newest circulation fan = Amazing. Also previously used a cheap crappy box fan blowing from top of tent downwards. Donāt overwater unless you like mold and donāt use crap cheap soil, also mold.
12/12 plants wonāt get BIG just medium at best (depending on the soil container size) but itās an easy set and forget schedule thatās in favor of your lighting bill costs.
Grow many plants in 12/12 to get a sea of green, otherwise Veg in 18/6 for 3 to 5 weeks and flip plants to get bigger harvests.
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