I have an 18 gallon plastic tank from an old water softener I removed for a client and built a pvc frame and taped plastic on it
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Added water and put an aquarium heater and air stone in and now I can pop seeds or root clones with little issue. It could be made to fit whatever size, within reason of course.
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I just stuck that solo in there to show the size. The smaller plants are clones and need to be potted but they're all wet from condensation and I don't have to add water very often.
A 5 gal bucket could be used or old aquarium. I built this one to help keep my sm greenhouse warmer in sunny so cal winters and it has really helped and made seed popping and cloning much easier. I remove the heater in summer?