Can i use Miracle-Grow Poting Mix for freshwater plants?

No, it has has alot of chemicals that are lethal to fish.

I wouldn't even recomend that for an all planted tank. The product was designed for non-submersed usage and I would be that it is a time release on the ferts in it. Putting it in water would cause a massive release of ferts and a subsequent algea bloom as the plants would not be able to use them all.

Enriched soils are absolutely inadvisable in planted aquaria. However there are soil recipes you can use that are perfectly safe for fish and shouldn't be any more risky with respect to algae than commercial substrates.

I am currently experimenting with a simple recipe using a mix of vermiculite and garden soil (regular top soil straight from the ground) covered with one inch of sand in submerged pots. I planted microsword and pygmy chain sword and both are showing new leaves after just one week. We'll see if it lasts without the emergence of algae but so far so good.

Other recipes have three or four layers of various materials including clay or "loam" soil, sphagnum moss and laterite. Regardless of the recipe they all use a material such as vermiculite or Pearlite to soak up nutrients and keep them in the substrate.

Try aquarium sand.

I've started using the aquatic plant soil that is sold at home depot. It looks disturbingly like kitty litter.

Aqua Soil is probably the best out there

Youe can try Florite or Eco-Complete.

I have had great results with eco-complete, and it looks awesome.