Hmm.. well I googled and the first link has many things to say about Miracle-gro:
"Miracle-Gro is a synthetic
fertilizer that contains ammonium
phosphate and several other chemicals that can be
toxic to your
soil and plants. ... The application has to be timed carefully and placed properly to avoid burning the leaves and roots . . . . In addition, ammonium tends to
inhibit the release of . . .
potassium
Miracle-Gro contains muriate of
potash, which contains excess chlorine that will burn plants and
inhibit the uptake of
nitrogen"
I see 2 big problems...
1) ammonium-phosphate: its really easy to overdose ammonium, and will also turn your pH very acidic very quickly. NH4+ + H20 <---> NH3 + H3O+ In addition to to the acidic water burning your plants, the elevated levels of phosphate will invite tons of algae.
2) muriate of potash: same thing as KCl. The potassium is good, but the chlorine is very very bad... Considering that KCl will dissociate completely, you will definitely have elevated levels of chlorine.
I'm not sure if Miracle-gro lists their chemical concentrations so dosing might be tough to figure out. My guess is that with a light dosing your plants will grow but you'll need water changes for additional dosing to prevent the toxic chemicals from accumulating too much. There's definitely better ways to do ferts but it might be an interesting experiment.