How to Reduce Nitrate in Your Aquarium

Written by:

PetMD Editorial
Published: April 09, 2019

Perhaps the most challenging part of maintaining aquaria (whether freshwater, brackish or marine) is controlling nuisance algae growth.

It seems the stuff just spontaneously generates if you make one wrong move, such as overfeeding the fish, skimping on a water change or slacking on replacing chemical filtrants.

Algae cannot simply grow from nothing. Yes, being capable of photosynthesis, they certainly do obtain their energy from light. But like any plant, they must obtain the basic building blocks of their biomass from their surrounding environment in the form of various nutrients. In other words, algae need fertilizers.

The most important components in "algae fertilizer" are ammonia/ammonium, nitrite and nitrate. Nitrate is the form preferred by most plants. Therefore, maintaining low concentrations of macronutrients, such as nitrate, is key to keeping an algae-free tank.

What’s the Role of Fish Tank Nitrate?

Because it is so important, nitrate can get used up rather quickly in the natural environment. In fact, in many ecosystems, it is a limiting nutrient. Nitrate is critical for the growth and reproduction of plants, and because it is in low supply, its abundance very directly influences the algal productivity. In the characteristically nutrient-poor waters of habitats such as coral reefs, algal growth is very, very restricted.

Fish tanks are another story, mainly due to the fact that conventional aquaria are recirculating systems that filter and recycle water over and over again. Though toxic substances are converted into less toxic substances, accumulations of their biological byproducts can themselves pose issues.

Take, for example, the nitrogen cycle. As we feed our fishes, they produce nitrogenous waste products in the form of ammonia. Nitrifying bacteria in our so-called biological filter convert the ammonia to nitrite and the nitrite into nitrate. All good, right?

Not necessarily! Where is all that nitrate going? If you cannot answer that question with a number, you need to start testing your water!

Reliable nitrate readings can be quickly and easily obtained using a quality test kit, such as the API Nitrate fresh and salt water aquarium test kit. If your nitrate levels are above, say, 10 or 15 parts per million (ppm), you've got some stuff to do.

The Potential Dangers of Excess Nitrate Levels

Sure, you've been told so many times that nitrate is harmless. A lot of fishes can tolerate brief exposures of up to 550 ppm. Chronic exposure, on the other hand, can indeed be damaging, even at much lower levels of exposure.

Over time, at just 30 ppm, nitrate can negatively impact cell development in both fishes and invertebrates. Lethargy, poor color, poor immune system and weakened feeding response are all signs of nitrate poisoning.

Most professional aquarists contend that nitrate concentrations should never exceed 20 ppm but are much more safely maintained below 10 ppm.

Still, nitrate concentrations of just a few parts per million can lead to massive algal blooms. These may occur as either planktonic (e.g., "green water") or benthic (e.g., film or slime) blooms.

By the time they become evident, they are already well on their way to choking out your corals, adversely altering your water chemistry and making your tank look generally overgrown and derelict.

 Truly, the best way to avoid battling algae in these endless cycles is to effectively prevent its proliferation through strict nutrient limitation. 

How to Reduce Nitrate in an Aquarium

There are essentially two manners in which nitrate levels can be kept low, even in well-stocked and well-fed aquaria. These are (1) minimizing nitrate input and (2) promoting its removal/uptake.

Minimizing Nitrate Input

Of course, very few among us would grab a bottle that boldly reads NITRATE and pour its contents into our aquarium. Nitrate gets into our tanks in more sneaky ways, such as in replacement water, supplements and fish food.

Consequently, one should use only purified water for replacement or top-off, be certain that any products used in the water are nitrate-free, and then, follow that big one: feed your fish sparingly!

Conducting Water Changes

Removal is simple enough if you carry out large, regular water exchange. Water changes are a sure shot, as they instantly and permanently remove the nitrate from the system.

Want to remove 20 percent of the nitrate in the water? Do a 20 percent water change; it's as straightforward as that.

Additionally, use of chemical filter media (such as the Deep Blue Professional nitrate reducer filter media pad) between water changes can provide very welcome relief in the event of an odd spike.

Installing a Refugium

For those who care not for tasks that require hauling heavy buckets about, there is one option that can significantly reduce the demand for water exchange. Using a planted refugium, the keeper can orchestrate the uptake of nitrate directly from the water via living, growing macroalgae.

Removal of nitrate (and other nutrients) occurs when portions of the standing crop are harvested and discarded. Though they require a little extra investment to install, refugia offer huge long-term payoff in the form of continuous, near-effortless and totally natural nitrate removal.

One might say that they provide the most interesting means of nitrate removal, in that cultivating the seaweeds (some of which can be quite beautiful) is a rewarding effort all in itself.

Using Microbes

Lastly, one might control nitrate levels (here again through biological action) using various types of microbes. These diverse microorganisms either sequester nitrate for biomass or convert it to another substance (e.g., nitrogen gas).

Live cultures are increasingly becoming available in the trade. These include both aerobic and anaerobic types. Aerobic forms (mainly heterotrophic bacteria) can take up nitrate rather quickly but usually need to be "fed" a carbon source, such as ethanol.

Anaerobic forms (such as purple nonsulfur bacteria) work more slowly but do not require carbon dosing, as they typically dwell deep in the bottom sediments where organic matter is plentiful.

Additionally, unlike their aerobic counterparts, anaerobes pose no risk of blooms (typically from carbon overdosing) that can cause dangerous oxygen depletion.

In most cases, it'll be best to foster a microbial community that is as dynamic and diverse as possible. This could involve the addition of a deep sand bed, use of bacterial inoculants and regular addition of bacterial supplements/foods.

However you choose to manage nitrate, one thing is clear: to keep healthy animals in a relatively algae-free tank, it must be managed aggressively.

By sticking to a tight water change regimen, using only purified water, adding quality chemical filters, installing a refugium and supporting the growth of beneficial microbes, you need not ever struggle with chronic nitrate build-up!

By: Kenneth Wingerter

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