Many thanks to Andrew (Mr. Ballroom) who directed me to the following web site for a recipie for making my own very low cost aquarium plant fertilizer. Fertilizer This link talks about experiments performed in keeping algae down but plant life growing. In case the page dissapears (It was there when I first linked to it in 1996, and was still there when I checked in 2017), here is the formula:
Note, if you choose to use this formula, do NOT use activated charcoal in any of your filters, the charcoal will remove the trace elements and defeat the purpose.
(Most of the ingredients can be purchased at hydroponics shops or garden supply stores. Epsom salts are available inexpensively at pharmacies)
Dissolve the trace element mix in 150mL distilled water, then add the remaining ingredients. Pour in additional water to make 300mL solution. The HCl helps prevent the growth of fungus and may be omitted if the mix is kept in the refrigerator. Add enough mix to the tank every day to keep the Fe level at about 0.1ppm (the exact amount will have to be determined by experimentation, but 3mL per 100L tank water is about right for a tank with rapidly growing plants). Measure nitrate levels regularly, and adjust the amount of KNO3 in the mix to maintain 3-5ppm (this step is fairly important). Those concerned about adding nitrates to their aquarium can dose the KNO3 separately, omitting it initially and adding it later as required to obtain the desired concentration.
The shelf life of the solution is unknown. Make small batches, or store only dry powders (but mix them with water before adding them to the aquarium).
If you don't like making up your own batches, let me know and I'd be willing to sell premade batches at a reasonable price.