Most planted tanks will require some fertilization at least now and then. For tanks that are light on nutrients and slow growing, buying pre-mixed fertlizer products, such as SeaChem's line of Flourish fertlizers, isn't prohibitively expensive. However, if you have a high-light, fast growing tank that sucks up nutrients rapidly, buying these products can quickly break the bank. This is where dry fertlizers come in.
Dry fertlizers are much cheaper and can be bought in bulk. They allow you to mix in different amounts of each nutrient to tailor it for your tank. However, using dry ferltlizers takes some knowledge of both chemistry and biology.
Plants need lots of different nutrients to grow, and aside from hydrogen and oxygen which they get from water directly, they mainly need carbon (from CO2), nitrogen, potassium, and phosphate (also known as macronutrients). These occur naturally in water to some degree, but in high-growth aquariums will be used up rapidly. When this happens, growth stunts and algae takes over. To comabt this, we use fertilizers.
The most common dry fertlizers are KNO3 (potassium nitrate), K2SO4 (potassium sulfate), and KH2PO4 (mono potassium phosphate). As you can see, these three provide the basic macronutrients. These can all be found readily online and quite inexpensively. However, it can be very confusing to determine just how much to put into your tank. This depends greatly on every single factor affecting the growth of the plants in your tank (light, CO2, other trace elements present, etc).
One way to figure out how much to use is to start off slowly and measure the levels of nitrates and phosphates to see if your plants use up what you put in. For example, test your water for nitrates, record the level, and then add a small amount of KNO3. Re-test the nitrates immediately afterwards and record the level. Then re-test the water in a day or two to see how that level compares to your first reading. If it's back to the first reading, your plants have used up all of the nitrates available, and you might need to add more next time to prevent the nitrates from "bottoming out."
An alternative to all this testing (which requires testing kits and lots of close observation) is to simply estimate and use a weekly water change to "reset" the nutrient levels. Use a fertilizer calculator, such as the Fertilator, to determine roughly how much your aquarium needs. Then, try it out for a few weeks, making sure to do the weekly water change to eliminate any excess. If you notice extra algae or stunted growth, you might need to adjust your routine. Test kits can help identify the culprit.
Either way you measure out your ingredients, you can either put the dry nutrients directly into your tank or create a pre-mixed solution (like the products available at fish stores). Either works well, however making a pre-mixed solution often makes adjusting individual nutrient amounts a headache, and often the solution will need to be refrigerated to prevent unwanted growth if not used quickly (it is a pure nutrient bath after all!).
The most difficult part of doing your own fertilization is probably figuring out how much to add. Whichever way you figure this out, once you have a routine down, you'll be saving money and have much more control over your aquarium!
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