Well, you could find a fertilizer tablet that is mostly KNO3 and push it into your substrate. If you have a source of pure KNO3, which many of us don't because of it's use in explosives, you could dissolve it in water then allow the water to evaporate, leaving KNO3 crystals which could also be pushed into the substrate.
If you have a plain gravel substrate the KNO3 will likely leech into the water and possibly do more harm than good. If you have a more or less watertight substrate, like soil, sand or even gravel with laterite then there will be next to no leeching of KNO3. Also, clay-rich soil, wether it's super-expensive laterite or plain old dirt with some clay content has a surface area 10,000 greater than sand (Don't even ask about gravel) and allows far more efficient transfer of nutrients to plant roots. Some clay in the substrate - not 100% clay; that would be too impervious to water.
If you have a tank with plain old gravel, your chances of growing plants are low. My dad had some sucess, but only by using much larger gravel than the average aquarium (about 5-10mm), and NEVER vaccuming his substrate. During the time his plants were getting established, weekly water changes kept his fish healthy. A layer of dead plant matter and fish waste accumulated on the bottom of his tank and plants rooted in it. Anerobic conditions were not an issue; plants bring enough oxygen into the soil to feed bacteria that oxidize Hydrogen Sulfide, a gas that is toxic to plant roots and fish, into water and sulfer dioxide, both of which are harmless. His tank is still doing fairly well. He has grown Valisinaria, Saggitarius, Aponogeton, Amazon and Melon Swords, Java Moss, Java Fern and Cryptocorynes in the tank. The only plant he could not grow sucessfully was Hygrophilia Polysperma. It was one of the first species he introduced to the setup, and it started growing like the weed it is, and died back just as quickly.
I would not risk adding ammonia in any form to the tank - NO3 compounds are not especially toxic to fish. Bass are known to live and reproduce normally in levels of >400mg/l. Levels this high would be a bad thing in an aquarium, but unless you are keeping delicate fish levels below 50 ppm are nothing to worry about.
The real cause of alage is usually excess phosphate and iron. Nitrates alone will not cause the problem. In many tanks with alage problems, unhealthy fish and high Nitrates, the nitrates are just a symptom of the real problem, overstocking or overfeeding. In the case of a beginner's tank, usually both.
/www.gardenchemicals.co.uk/pdt_potassiumnitrate.htm
