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Starting a Planted Tank

Plant Nutrients 101

Phophates in the Planted Tank

Activated Carbon

Fertilizer & the Planted Aquarium

CO2 & the Planted Tank

Algae Control

Safety Around the Aquarium

Cleaning Aquarium Glass

Mysterious Fish Deaths Explained!

Aquarium Photography

New Tank Syndrome

Choosing an Aquatic Heater

Tips for Beginning Fishkeepers

Salt in the Aquarium

Outdoor Patio Tubbing

Malaysian Trumpet Snails




The one chemical in the aquarium that is sure to cause frowns and wrinkled faces is Phosphorus. 

Phosphorus can be good AND evil, depending on the circumstances.

When present in excess quantities, Phosphorus tends to cause unwanted – and unsightly – algae growth.  It seems that whenever algae begins to overwhelm a tank, it is always a matter of nutrition – for the algae!  Algae is similar in its requirements to our desired aquarium plants.  It consumes the same nutrients:  Iron, Magnesium, Potassium, Phosphates, etc.  But when all nutrients are present in excess quantities (i.e., there is more in the water column or in the substrate than necessary for the plants’ immediate requirements), then this is frequently followed by an unwanted increased growth of algae. 

Algae can occur in many different varieties.  There is green spot algae, which deposits itself on plants and on the glass.  There is hair algae, which seems to be particularly difficult to control and eradicate.  These two algaes seem to be the most prevalent ones, and the ones that most frequently draw complaints from aquarists.

Where do nutrients come from?  Why, we add them, of course.  Aquarium plant fertilizers contain most or all of the nutrients required by plants.  However, most aquarium plant fertilizers deliberately omit Phosphorus (Phosphates) because they tend to encourage unwanted algae growth.

If we test our aquarium water, we still often find the unwanted presence of Phosphorus.  But since we didn't add it with the fertilizer, where did it come from?  Most aquarists then test their municipal water supply, and occasionally in some parts of the country, Phosphorus is present in higher than desired concentrations.  (Anything beyond barely detectable is usually considered higher than desired.)  But most cities precipitate out Phosphates from their water because they also don’t want algae growing in their water distribution lines throughout the city.

So, if it’s not coming from the fertilizer, and it’s not coming from municipal water, what other source of Phosphorus can there be in the aquarium?

Fish food.

Fish food is formulated and balanced to meet the nutritional needs of fish.  Fish require many nutrients to maintain their health.  They require proteins to repair and build muscle tissue.  They require fats for energy, and to make the fish food taste good to them.  And they require Calcium and Phosphorus to build and grow their skeletons.

Most good quality commercial fish foods contain an average of approximately 0.5% - 1.0% Phosphorus.  A few contain more, some others less; you can look on the fish food label to see exactly how much Phosphorus is included in your particular brand.  (A glance at my favorite brand of fish food shows that it contains 0.6% Phosphorus.) 

OK, so the fish are taking in Phosphorus.  And they are using some, and they are discarding the excess via their waste.  And an interesting phenomenon occurs in our fish:  they tend to concentrate the Phosphorus in their waste.  

I am unaware of any specific chemical analysis of Guppy poop, but commercial fisheries that raise agricultural and game fish have done these analysis.  And it stands to reason that their findings can be applied roughly to the content of the waste of tropical fish. 
Trout are raised in large quantities in commercial hatcheries, and they have been extensively tested and analyzed, since many of these Trout are destined to become food for humans.  And analysis of Trout poop shows that Trout produce a poop containing an average of 2.6% Phosphorus! 

That’s a lot of Phosphorus, no doubt about it.  Fortunately, if we maintain light quantities of fish in our planted aquaria, the total volume of Phosphorus will usually not be too excessive.  But it IS a consideration.  Additionally, if we feed excessive amounts of food to our fish, the excess will settle into the substrate and into the filter media, where it will decompose and contribute its ingredients to the water column.  Worded another way, if you overfeed your fish, you’re going to contribute to excess waste buildup (including Phosphorus), and you’ll likely be faced with an algae problem some day. 

OK, so we know that Phosphorus in aquariums comes from fish food, fish waste, and occasionally is introduced from our municipal water.  This is not usually a huge source with well-maintained city water sources, but it CAN become a problem if you get your water from wells, particularly those near agricultural operations.  Farmers spread a lot of NPK fertilizer on their fields, and the P in NPK is Phosphorus.  Since farm wells are not processed as are municipal water sources, you will find a few wells that introduce a lot of Phosphorus to aquariums.

Is this a problem for your planted aquarium?  Well, maybe yes, maybe no.  (Don’t you love definitive answers?)  All aquarium plants consume a lot of nutrients, and if you have sufficient numbers of healthy, hungry plants, they may be all you need to consume all the Phosphorus in your aquarium water.  The more plants you maintain, the more likely this will be true.  Heavily planted aquariums tend not to have significant problems with algae because the plants out-compete the algae for all nutrients, including Phosphorus. 

It is unlikely that you will ever be able to eliminate ALL algae from your planted aquarium, but it IS possible to control it.  Maintain as many plants as you can in your aquarium, and keep them growing briskly by feeding them quality nutrients such as Yamato Green (you KNEW there was going to be a commercial message in there somewhere, didn’t you?).   Don’t overfeed your fish (always a wise thing to avoid), and perform regular weekly water changes (to prevent buildup of waste byproducts and to remove fish waste before it has a chance to decompose and contribute its Phosphorus to the water column).  Since excess fish food frequently becomes trapped in filter media, be sure to clean the filter media regularly to prevent the waste materials from releasing their contaminants back into the water.  If you use Granular Activate Carbon (GAC) in your filter (not recommended if you also add plant fertilizers), choose a high quality brand that does not leech Phosphorus back into the aquarium water.  Best bet:  don’t run GAC at all.  It just pulls your desired nutrients from the water column and can starve your plants for food. 

If Phosphorus continues to show up in excessive amounts (more than 0.25% or 0.5%), try running a Phosphorus pillow in the filter box.  A Phosphorus (or Phosphate) pillow will pull excess Phosphorus from the water column.  Be sure to follow the product instructions, and change the pillow as instructed so that it doesn’t exhaust and allow Phosphorus to build up in the water once more.

To summarize:  choose a high quality aquarium plant fertilizer (of course we recommend Yamato Green) that does not contain Phosphorus as one of its ingredients.  If you find two fish foods that appeal to your fish, choose the brand that has the lower Phosphorus level, and then don’t overfeed.  Keep as many growing plants in your aquarium as you can.  Perform regular water changes.  Use a Phosphate pillow only as a last resort.  If algae still continues to be a problem, check your Phosphorus levels with a good quality test kit, and also check your municipal water source to see if perhaps you might be introducing Phosphorus via the water.  If you do all these things, algae should be a very minor problem for your aquarium.  The algae cleanup crew (SAE’s, Amano Shrimp, Malaysian Trumpet Snails, American Flag Fish, etc.) should be able to handle whatever manages to survive.

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