Most contain urea (listed on back as ammonical nitrogen sometimes), which breaks down into ammonia.
And a good portion contain heavy metals, which are toxic as well. Check the contents of the one you want to buy here
UDGags is absolutely correct regarding the heavy metals.
I use Miracle Grow Water Soluble General Purpose for my emersed growth plants but I will not use the product in my aquariums because it contains: Arsenic, Cadmium, Cobalt, Mercury, Molybdenum, Nickel, Lead, and Cobalt (Co). I value my fish (or invertebrates) too much to do that.