Check out:gpodio said:Urea is another nitrogen source however both of these alternatives could possibly cause some issues with algae growth. I think I'd stick to KNO3 personally. Others might have better alternatives.
Urea is basically NH4+, I think this reaction is pretty fast in our warm bacteria filled tanks. A quick lesson in hand waving redox chemistry might be in order here...
NH4 and Urea are prone to oxidation by oxygen. This means they are a "fuel" as they are very rich in electrons - they pack a whallop so to speak. NO3 is the leftover after the electron energy has been extracted by aerobic bacteria...
What does this mean practically? Well, nitrate is relatively a low grade of nitrogen. Because of this, it is unlikely to cause excessive algae growth, especially in the presence of healthy plants. NH4 and Urea however can cause explosive growths of algae very quickly.
If you're going to add urea or NH3 you need to deliver it where the algae aint, this basically means via substrate additions. You can do this in the form of Jobes spike, Osmocote, or in a more subtle way... it may be possible that in a deep enough substrate which is anaerobic to run the biofiltration reaction in reverse, that is convert NO3- back to NH4+...