Thanks for the info and advice
@myke 
I thought about putting the bigger container on top for soil, but those "shoulders" around the top 3 inches is where they stack onto each other. Can only stack same size, or smaller into bigger.
Could run 2 18 quarts together, would give you almost a gallon for the reservoir. Overall height would be the same as a 12 inside an 18 (15")
I'm hoping with the larger reservoir I can leave the plants for a longer period of time. Here's where I'm at right now.
View attachment 5009786
Picked up some Dr Earth liquid nutrients in a little pump bottle. I figure with only 3 gallons of soil, the plants will be hungry after the initial dry ammendments are consumed. Still planning to top dress when I flip to flower, hoping this will carry me through.
View attachment 5009818