Can I just throw my two bits in about the chemistry of this? Nitrogen gas dissolved in a liquid does nothing - it is completely inert. Ammonia (NH3) is toxic to fish, much more than a related compound, ammonium (NH4+). If you add any ammonia to water, a portion of it will turn into ammonium by reacting with water molecules.
NH3 + H2O <---> NH4+ + OH-
This reaction can go either forward or backward. In a basic solution (such as pH 8, there is already plenty of OH- around so the equation tends to go to the left (more ammonia). In an acidic solution (such as pH 6, there is very little OH- around, favoring the reaction going to the right (most ammonia converts to NH4+). The reaction is also dependent on temperature, but not as much.
To be toxic to fish, you need a certain concentration of ammonia in water. One of the above links gives some good ballpark concentrations. You can get ammonia toxicity even at a pH of 5 if you add enough (very rare, but could be done with enough windex, for example). For comparison, a very small amount of added ammonia would produce the same toxic concentration at a higher pH.
BTW, none of this has anything to do with potassium bicarbonate, which is the title of this thread.
Plants love NH4+, using it as their preferred source of nitrogen when it's available. As soon as molecules of NH4+ are removed by the plants, the above equation shifts a bit (to the right), and more NH3 is converted to NH4+. In a sense, it 'looks' like the plants can use either NH3 or NH4+ since they both exist in equilibrium in any solution.
Plants are also perfectly capable of using NO3- as a nitrogen source and it's much better tolerated by the fish (as we all know from the nitrogen cycle in non-planted aquariums.) Per some research done by Tom Barr, it seems that NH4+ has a detrimental effect when it comes to algae (exactly why has not been explained to me). In a healthy planted tank, the ammonia made by the fish should disappear quite quickly into the plants. Measurable concentrations of ammonia should be zero. Reasonable additions of NO3- don't seem to cause the same algae blooms, but still make Nitrogen available to the higher plants.
One of the readily available commercial fertilizers is ammonium nitrate (NH4NO3). Lots of nitrogen to go around..... It should be easy enough to find if someone wants to experiment with it. For that matter, pure ammonia isn't that hard to find either. Personally, I'll stick with my KNO3.