Plants can be easily grown in the aquarium if they receive a few basic necessities. These are light, carbon dioxide, and nutrients. The fish can produce some of the carbon dioxide and nutrients needed by the plants, but to grow the best plants you must balance the proper ratio of nutrients. A typical aquarium will have an abundance of nitrogenous and phosphoric nutrition for plants, but it is sometimes necessary to add supplemental trace minerals.
Among those that aquatic plants sometimes lack are iron, potassium, and magnesium. There are others that are required in much smaller amounts, such as zinc, copper and manganese. Most of these can be added very cheaply by using items sold in drug and garden stores.
If you purchase Tera's FloraPride, a 100 ml bottle will cost you roughly $7.00. This contains iron, approximately 0.02%, in the form of ferric sulfate. It also contains 3% potassium sulfate. It makes no claim to contain magnesium, zinc, or copper, which are required in smaller quantities. This 100 ml bottle will treat about 1000 gallons of water.
A much cheaper solution is to go to your garden supply store and buy a bottle of Ferti-lome's Chelated Liquid Iron. This is sold for treating garden trees and other plants for micronutrient deficiencies. Since it is chelated, it will stay in solution longer, and therefor be more available for the plants to use. A comparison in price is as follows:
A 1000 ml (one liter) bottle of Liquid Iron sells for about $7.00. This contains 3.25% iron, in the form of ferrous sulfate. It also contains copper, as copper sulfate, at 0.05%, and manganese, as manganese sulfate, at 0.15%, as well as zinc, as zinc sulfate, at 0.16%. If you adjust for concentration, the 1 liter of 3.25% iron would make 1625 bottles of the 100 ml 0.02% solution. That is a savings of about $11,368.00, if you ever actually use all of it. This bottle is more than a lifetime supply. But it is too concentrated to use by itself, and must be diluted properly. The best way to dilute it is to get a syringe that is marked in mililiters. Add 10 ml to a two liter bottle filled about 3/4 full of water. To keep it in chelated form, dissolve one 250 mg tablet of non-chewable vitamin C in the water. To add magnesium, add 1/8th teaspoon of epsom salt (magnesium sulfate) to the mixture.
There is still one element missing, and that is potassium. The ideal compound is potassium sulfate, but I have not yet found a good over-the-counter form of potassium sulfate. The next best thing is potash, which is sold at garden supply stores. This should not be added to the mixture above, because it will raise the pH and cause the minerals to precipitate from solution. It should be dissolved in a seperate container, about a 3% solution. Be sure to check the strength of the potash that you are using, and be sure that it doesn't have added nitrogen or phosphates or you will have algae problems. Add 10 mls of each of these solutions per 15 gallons of water when doing water changes, which should be at least once a week or more.