Biological Filtration and Saltwater Tanks

Saltwater Aquariums 101

Biological Filtration and Saltwater Aquariums

What is biological filtration?

A properly working biological filter is based on nitrifying bacteria and it’s ability to breakdown the bio-load of the aquarium’s livestock. When doing so, the biological filtration makes the water less toxic for marine fish, Invertebrates and coral. How effective your beneficial bacteria is, depends on the total surface area as well as how stable the bio-load is within the aquarium.

If you have ever heard of the term “cycling an aquarium.” This is basically referring to growing your biological filter. When you start a new saltwater aquarium there is very little (if any) nitrifying bacteria present in aquarium. To get an aquarium cycled you need to get your nitrifying bacteria growing. Nitrifying bacteria needs three things to do this surface area, bio-load and time (30-45 days in most cases).

There are products out there that can make this process instantaneous, but that’s for another story.   …Ok, If you can’t wait check out Dr. Tim’s ONE and ONLY Click here to visit DrTim’s Aquatics, LLC. But I will be doing a full review of this product and others down the road.

The key to a great biological filter is surface area.

I’m a naturalist at heart and I am a big fan of the Berlin Method when it comes to keeping saltwater aquariums. The Berlin Method is based on the use of live rock within the aquarium (it’s not alive but there are many living organisms that thrive on and with in it). Live rock is very porous which supplies nitrifying bacteria with a huge amount of surface area.

There are other, man-made forms of biological media available on the market today, but in my opinion they don’t even come close to replacing live rock.

Aquarium humor

Keeping your bio-load in check.

Every time you feed your fish, introduce new livestock to your aquarium or even perform aquarium maintenance, you are sending your biological filter into sudden-death …ok, too much coffee. The point I’m trying to make is that these things do negatively impact your biological filter. It’s not going to make your tank crash but it takes time for the aquarium’s nitrifying bacteria to catch back up.

Note: In a fully cycled aquarium Ammonia (NH3) levels should be at 0 ppm as well as Nitrite (NO2) levels at 0 ppm. Visit my post on saltwater aquarium parameters for more information on that subject.

A few tips to keep your bio-load in check:


Overfeeding: When feeding the aquarium, it is important to feed only what the fish and invertebrates can consume within a short period of time. Any uneaten food needs to be removed. If not, the uneaten food can release ammonia into the aquarium. This fluctuation in the bio-load and will cause an imbalance in the biological filtration.

Adding livestock: In established saltwater aquariums, there is only enough nitrifying bacteria to handle the bio-load of the aquarium and it’s livestock. When you add livestock to the aquarium, You’re chaining the bio-load of the aquarium. How high the toxins will become in the aquarium will depend on the amount of livestock added. If too much livestock is added at one time, it is possible for toxins to reach dangerous levels. It is important to stock the aquarium slowly over time, giving the biological filtration time to catch up to the bio-load.

Aquarium maintenance: Water changes can have a negative impact on the biological filtration to some degree. If your water is not properly filtered through a RO/DI unit, the chemicals and metals present in the water and can kill off the bacteria when added to the aquarium. No water changes greater than 25% of the total volume should be done at one time.

Note: If you’re in a position where you need to get the aquarium water parameters under control, it’s best to do a few smaller water changes over the course of a few days.

Biological Filtration Conclusion.

Biological filtration paired with mechanical filtration makes it possible for our aquariums to thrive. Maintaining a healthy saltwater aquarium starts with understanding these aspects and putting them to work. It’s important to give the biological filter the support it needs and it will be mutually beneficial. Remember: Too much, too fast, will make your tank crash.

Save a reef  …grow your own.

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