Ammonium nitrate, ammonium phosphate, and ammonium sulfate are popular nitrogen sources for terrestrial gardening, but you would have to be rather careful with dosing it to avoid fish toxicity from the ammonia it contains. Urea is another nitrate source, but it decomposes to ammonia, causing a slower-reacting potential for issue. This is why we tend to preferentially use potassium nitrate in aquariums.
Copper sulfate and Zinc sulfate are also popular in fertilizers, but can be quite toxic to fish compare to the chelated forms typically used in aquariums.
There are any number of other problematic compounds, but these 6 pop to mind right away as potentially hazardous to fish.
That said, some slow-release granular garden fertilizers are used by quite a few folks for substrate fertilization. Osmocote plus contains 4 of the above 6 (no ammonium sulfate or urea), but is often used for DIY root tablets. Being used buried deep in a substrate and having a slow-release coating mitigates the problems somewhat. Some do have issues with these, some do not.
Last edited by mattinmd; 11-26-2014 at 01:20 PM.
Reason: forgot to count urea...