[learn2turn]

Which Miracle Grow soil? there's a whole bunch --
http://www.scotts.com/smg/catalog/productCategoryLanding.jsp?navAction=jump&branPage=miraclegro&itemId=cat50150&id=cat50006

The styrofoam that you took out was probably perlite which would likely be in a potting soil and not a garden soil so you might be using potting soil.

They all likely have fertilizers. A lot of horticultural produces use urea for N (because it's cheap) and that might not should up on your tests until bacteria convert it to something else. But, maybe somethings eating it as it's converted.

I probably know less than anyone here about this stuff but I'd select some other soil next time and in the meantime do lots of water changes and hope the problem goes away.

 

[learn2turn]

Oh, my intuition would lead me to believe that cheap plant fertilizer is very bad in aquariums.

Remember I said most fertilizer uses urea for N because it's cheap. It's not available to plants till it processed by bacteria in soil. That takes a few days to a few weeks and it's no big deal for your begonias. I used to grow orchids and know that orchid fertilizer shouldn't be urea-based as there's no conventional soil to break down the urea.

Now, guess what urea is? The same stuff in animal waste. Bacteria goes to work on it just like your fish poop.

(NH2)2CO -> NH3 -> NO2 -> NO3

So if you add urea-based fertilizer, you are hugely increasing the bio-load on your aquarium. That could cause your NH3 or NO2 to raise.