line the inside of your cupboard with mylar. Mylar is a highly reflective material that will reflect about 98% of the light given off back at the plants without burning them, its a must. Also, what your going to do is put a inlet fan on the bottom side of the cupboard, this is going to supply you with fresh air. At the top on the opposite side should be another fan, as an outlet to take the dead hot air out. When the fresh air comes in, its going to be packed with CO2, which is what plants need. As the air is processed by the plants, it will become dead air with very little CO2 content. The air is going to heat because its in the cupboard, and as we all know, hot air rises, so thats where your outlet fan comes in. Dont worry about soil with nutes in it, and in all honesty if you are looking to grow good cannabis, i wouldnt even bother with any miracle grow. Next time you do it, go to your local gardening store, find a bag of good soil and then get a bag of perlite and make your mix. If your going to use nutes, use the slow release nutes on the plants such as bat guano (highly recommended).
You were right about the water, if you leave it out for a 24 hour period, it will distill the water, but if i were you, id go buy the pH tester and then some pH down, or vinegar (both have same effect). pH of water should be around 6 (shows up as a yellow).
When watering your plants, you should be using about 1-2 gallons a week, let the soil COMPLETELY DRY before you water again, if done correctly this will prevent root lock and your plant will be healthy as a horse.
Also, one last thing, if your looking to really boost these plants, get a CO2 setup. If theres any other questions let me know, id be happy to do my best to answer any and all.
Happy blazin fellow botanists
