Fertilizing Aquatic Plants

Perhaps one of the more confusing aspects of keeping a lush planted tank is knowing what and how much to fertilize. Adding to this confusion is the popular belief that adding too many fertilizers will cause algae problems.

First of all, what do plants need? All plants need the following things to survive:

The three biggest components (other than carbon, or co2):
Nitrogen (nitrate, or N)
Phosphorus (phosphate, or P)
Potassium (K)

And in lesser amounts:
Calcium (Ca)
Magnesium (Mg)
Iron (Fe)
many other elements often found in a “trace mix”

Now you might say – “I don’t add any of these things to my planted tanks, and my plants are fine.” Well, there are many ways that plants get these nutrients. One major way is through waste from the fish. As you probably already know, one of the major byproducts of fish waste is nitrogen (ammonia, nitrate, or nitrite). Fish food (and therefore fish waste) also contains phosphorus, potassium, and many other substances that plants need. Also, your tap water probably contains many of these substances – for example, GH is a measure of calcium and magnesium. If you have a GH of more than about 4 degrees or so, you probably already have plenty of these two elements to keep plants happy. Many other things exist in tap water that we don’t test for, that are probably already feeding your plants.

So why fertilize at all?

The biggest problem occurs when plants are growing and using nutrients faster than they are being replenished. A plant will create it’s own reserves of some nutrients when there is an excess, but if it’s not replenished fast enough to account for growth, at some point it will become deficient.

Then what happens? Often one of the first signs is ALGAE. Algae can be opportunistic, growing on plants that are deficient in some nutrient and perhaps in the early stages of death. Other signs are spindly growth, holes in leaves, leaf loss, and just overall plant death. It’s true that other conditions can cause these symptoms too, so it’s important to evaluate your own setup to determine whether nutrient deficiencies could be the problem.

One important thing to understand is that light FUELS growth, as does CO2 injection. If you have low light and no co2 injection, your plant’s needs might already be cared for adequately by your fish waste, substrate, etc. But add higher light and/or co2, and the reserves of nutrients could quickly become diminished. Therefore, it’s usually NECESSARY to fertilize in any co2 injected tank.

It also can become necessary even in low light tanks if algae becomes a problem. This may seem contradictory to some of the advice out there that says that algae is due to having too many nutrients in the tank. This is true in a non-planted tank situation – there is no need for nutrients, so the simplest way to reduce algae is to eliminate the nutrients. But in a situation with plants, if you eliminate the nutrients, the plants will die along with the algae, in fact the deficiencies this will create in the plants will only serve to fuel algae even further as the plants die off. So what you have to do is eliminate the RIGHT nutrients – the ORGANIC ones – i.e. fish waste, debris rotting in the substrate, etc…. and then ADD the INORGANIC nutrients by dosing fertilizers. Organic nutrients and waste breaking down in the tank/filter will fuel algae like nothing else – possibly in part because it produces ammonia which is a PROVEN way to induce algae in a tank. So if you see algae making an appearance – the first question to ask yourself is – have I been doing my regular weekly water changes? And the second is – have I been fertilizing properly?

So what do you add and how much? There are many commercial fertilizers on the market. Many of these make big promises but when you read the label, you’re not really getting what you need. This is why I use individual, powdered fertilizers – they’re cheap and I know exactly what I’m adding to the tank with every dose.

To go this route, you need:
KNO3 (to add nitrate and some potassium)
K2SO4 (to add potassium)
KH2PO4 (to add phosphate)
A trace mix such as Plantex CSM +B (adds iron and traces)

And also GH booster, or calcium chloride (pool hardness increaser) and magnesium sulfate (epsom salts) if your GH is below 4 degrees.

With those items you have virtually everything you need to fertilize with. There are various places that sell them, such as www.rexgrigg.com. A years supply for most people will cost you less than $20.

Next you need to figure out how much of each to dose. The higher the light and co2, the more you will need to add on a regular basis. You can either start small and work your way up, or you can dose according to suggested weekly amounts and do a 50% water change at the end of the week to reset amounts so they never get exceedingly high. Download my fertilizer calculator if you need to figure out how much of each thing to add.

http://www.beginneraquarist.petfish.net/Beginner Aquarist/Fertilization.html

The bottom line is: Fertilizing is one of the cheapest and easiest things you can do to ensure plant health and keep algae at bay. For the cost of a few plants you can fertilize your tank for a year or more. And there is virtually no risk. Why not start today?