I've never tried leaf propagating an aloe -- I think it's easier with pups as mentioned above or when you already have the basal tissue of the leaf attached.
Assuming water propagation is possible at all for aloe, I believe you would need to let the cut end dry and form callus first.
When water propagating, it's essential to change the water frequently and not allow microorganisms to breed -- I think adding MircleGro to the water might have fostered unwanted harmful growth -- fungi, algae, bacteria, etc. in the water.
I would try with several dried/callused leaf cuttings in moist sand or 1/2 and 1/2 sand and potting soil as propagating medium and put a vented humidity dome on top. Bright, diffused light. Bottom heat.