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Undergravel filter is an internal aquarium filter that well-known provides the best biological filtration for your fish tank. Of course, to achieve the best results you need to make sure that you are using the right aquarium air pump or powerhead (water pump) and have the right amount of gravel to set up an undergravel filter. So, this article will provide you these basic knowledge about this powerful filter as well as how to set up and the best undergravel filter for your aquarium.

How does an undergravel filter work?

An undergravel filter should be set up when you are initially starting an aquarium, being that the majority of this system resides beneath the substrate. On the bottom of the aquarium, you will place one more plates (depending on the size of the tank). The gravel substrate is placed on top of these plates, allowing water to flow freely beneath the substrate. The filter tubes connect to the plates at the bottom of the tank and rise up to the top where the filtered aquarium water exits and returns to the tank.

Penn Plax undergravel filter for 20 gal tanksPenn Plax undergravel filter for 20 gal tanks

These filters can be powered by 2 methods, air pumps or power heads:

Air Pump – A fish tank air pump is used to push air through a line down to the bottom of the filter tubes, the air bubbles rise to the top of the tube pushing water out with the air bubbles and by effect water is drawn trough the gravel to the bottom of the tank. If you go this route, do the smart thing and make sure you put a check valve in the airline. Any air pumps that I have seen will tell you how many gallons they are rated for, be sure to buy an air pump that is rated for the volume of your aquarium.

Powerhead (water pump)Water is pumped up and out of the filter tubes back into the aquarium and, by effect, drawing new water through the gravel to the bottom of the tank. Not a whole lot different from using an air pump. The powerhead you should be using depends on the square footage of your tanks base. Your powerhead should be rated from 60-90 GPH per square foot. For a 16×30 inch base you should be looking at a powerhead that will pump 200-300 gallons per hour.

This is how the cycle flows no matter what you use to move the water. Using an air pump will add a little more oxygen to the water, whereas a powerhead will likely move the water faster increasing the number of gallons per hour that are passing through the gravel.

Three filtration types in an undergravel filter

The gravel itself functions as your mechanical filter. As water flows through the gravel, any larger particles will be caught by the substrate allowing you to clean it from the aquarium during water changes, via a siphon (python) or an aquarium gravel vacuum cleaner. So cleaning an undergravel filter maybe not too hard as many people think about this filter.

aquarium-gravel-cleaning

Clean the gravel with a vacuum cleaner

The biological filtration takes place in a few key locations. The gravel, the grates beneath the gravel, the bottom of the aquarium, and the walls of the filter tubes. All of these places cultivate bacteria and aids in removing the ammonia and nitrites as the water flows through the cycle.

Though it is a good filtration system for biological filtration, okay for aiding in mechanical, and can incorporate some degree of chemical filtration, most aquariums should use this as an additional filter with a power, internal, or canister filter doing the majority of the work.

Some systems have a place for a carbon insert where the water exits the lift tube. Not all undergravel filtration systems have this chemical filtration component and it is not a big deal if you are planning to have a more advanced filtration system in place along with the undergravel filter. If the undergravel filter is the only type of filtration you intend on using, I strongly recommend that you make sure the chemical filtration stage is part of the setup that you purchase.

Pros and cons of undergravel filters

  • Very inexpensive
  • Won’t suck up very small fish and invertebrates
  • Provides supplemental mechanical and biological filtration
  • Optional chemical filtration stage (minimal)
  • Maintenance free without chemical filtration / carbon
  • Very simple maintenance if using chemical filtration (Change the carbon during your regular water changes every 3-4 weeks)
  • Easy to setup (as long as it is done when you start the aquarium).
  • Needs to be installed during the initial setup of the aquarium
  • Limits substrate options (no sand)
  • On its own, this aquarium filter system will not be adequate for most aquariums
  • Air pump or Powerhead needed but not included.

Best undergravel filters for various tank sizes

Penn Plax CFU-10 for 10 gallon tank, 9.5 by 19-inchPenn Plax CFU-10 for 10 gallon tank, 9.5 by 19-inch Penn Plax CFU-20, 11.5 by 23 -inch for 15/20H gallon tankPenn Plax CFU-20, 11.5 by 23 -inch for 15/20H gallon tank Penn Plax CFU-29 for 20L/29 gallon tank, 11.5 by 28.5-inchPenn Plax CFU-29 for 20L/29 gallon tank, 11.5 by 28.5-inch Penn Plax 40/55 Gallon Under Tank Filter, 11.5 by 46-inchPenn Plax 40/55 Gallon Under Tank Filter, 11.5 by 46-inch Lee's 5-1/2 Premium, 8 by 16-inchLee's 5-1/2 Premium, 8 by 16-inch Lee's 10 Premium, 10 by 20-inch for 10 gallon tanksLee's 10 Premium, 10 by 20-inch for 10 gallon tanks Lee's 30 Premium, 12 by 36-inch for 30 gallon tanksLee's 30 Premium, 12 by 36-inch for 30 gallon tanks Lee's 40/55 Premium, 12 by 48-inch for 55 gallon tanksLee's 40/55 Premium, 12 by 48-inch for 55 gallon tanks Lee's 50/65 Premium, 18 by 36-inchLee's 50/65 Premium, 18 by 36-inch Lee's 70/90 Premium Undrgravel Filter 18 by 48-InchLee's 70/90 Premium Undrgravel Filter 18 by 48-Inch

How to set up an undergravel filter?

People like to use air pumps and not powerheads with their undergravel aquarium filters so I don’t want to make any recommendations as to which one to use with your system or where to buy one, though I’m sure any quality powerhead would work just fine. It is very easy for you to assemble an undergravel filter. After the assembling, just connect the hoses of the filter with an external air pump using air hoses and accessories.

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You should set up the undergravel filter before adding gravel in a fish tank setup. 1.5 – 3 inches of substrate is the general recommended range but of course the amount you need depends on the size of your aquarium. I recommend staying closer to the inch and a half end of the range for 10 – 20 gallon aquariums and aquariums that are 100 gallons or so should have closer to 3 inches of gravel. It doesn’t have to be an exact measurement, many types of fish will move the gravel around anyways.

Do I need an undergravel filter?

The factors determining whether or not an undergravel filter can be used by itself in your aquarium include: frequency of water changes, how heavily your tank is stocked, and even what kind of fish you are keeping. Some fish are messier eaters than others and some fish seem to produce more waste, i.e. Plecostomus.

I only recommend this filter if you are planning for a less maintaining aquarium with several fish for the large volume of the tank, or you want to use this as an additional filter to best perform biological filtration in your aquarium.

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