What to Know About Undergravel Filters

Coral tank with no substrate for undergravel filter

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Undergravel filters are a type of aquarium filtration that is located beneath the substrate (or under the gravel), and using them avoids the need for a clunky filter hanging on the back of your tank. There is some controversy about undergravel filters, for several reasons. Although these filters eliminate a dirty maintenance routine of frequent filter cleaning, once they are in your tank and the substrate and decor is placed, you cannot access them. Without proper maintenance, undergravel filters can cause aquarists considerable issues.

How to Best Utilize an Undergravel Filter

If you plan on using an undergravel filter, keep the following tips in mind:

  • Use a coarse gravel substrate (no sand)
  • Gravel vacuum the substrate regularly
  • Keep your bioload low (number of live components)
  • Select one with a rigid undergravel plate
  • Use enough filter plates to cover the bottom of the tank
  • Cover the filter plate with 1-2 inches of gravel

How Undergravel Filters Work

To install an undergravel filter, you must start with an empty tank. The filter is made up of a flat, rectangular plastic plate that will sit on the bottom of your tank. There are uplift tubes attached to the plate for moving water using either an air pump or a powerhead water pump. The tube pulls water down through the gravel and under the plate, then back up the tube. This creates a water flow through the gravel that pulls waste down and allows for aerobic bacteria to grow in the gravel, making the entire gravel bed a biological filter. Waste and debris will fall through the substrate and under the filter plate. The debris remains under the plate, and needs to be periodically removed by deep gravel vacuuming using a siphon.

The undergravel filter plates should cover the entire bottom of the tank, so more than one undergravel filter plate is needed in a larger tank. This will allow you to have suction from multiple areas to pull water through the gravel and under the plate. The suction is created by using an air pump attached to an air stone down the uplift tube. The bubbles from the air stone should be fine with a strong flow, to pull water up with the rising bubbles. That creates suction with water going down through the gravel, under the filter plate, and up the tube with the bubbles. Alternatively, there are water pumps called "power heads" that are made to sit on top of the uplift tube and pull water through the filter plate. These are stronger and more efficient than using air stones.

Some undergravel filters will include cartridge components—typically containers of activated carbon or foam sponges—that attach to the uplift tube spout. You do not need to use these in your undergravel filter, except for the few setups listed below.

Main Issues With Undergravel Filters

Other than maintaining a better aesthetic, there are few benefits for undergravel filters. Because they lack access, once you put it in your tank, you will not be able to clean it fully unless you break down the aquarium completely to clean it and start over. Undergravel filters can trap massive amounts of debris that cannot be removed without removing the filter if they are not regularly gravel vacuumed. Hanging on the tank or canister filters, although unsightly and messy, are easier to keep clean and maintain regularly. Excessive amounts of debris accumulating in the gravel can elevate the ammonia and nitrate levels.

How to Maintain an Undergravel Filter

In order to maintain your undergravel filter, you will have to keep the substrate clean with regular gravel vacuuming because this filter pulls debris into the gravel bed. If you don't regularly clean your substrate, all the debris is pulled under the filter plate, and it will be a haven for muck to settle. One way of cleaning the undergravel filter plate is to attach the inflow tube of a canister filter to the uplift tube. Place the outflow tube into a bucket and turn on the canister filter. This will suck all the accumulated debris out from under the filter plate and into the waste bucket, which effectively cleans the filter and does a water change at the same time. By limiting the amount of debris that ends up in the gravel and under the filter plate, you will prolong its function.

Always ensure that your undergravel filter fits snugly in your aquarium. If there is space on the side that lacks a filter, lots of debris will get trapped in these spots.

Types of aquariums best suited for undergravel filters

The following types of aquariums are the best choices for undergravel filters.

Hospital/Quarantine Tanks

Hospital and quarantine tanks are only used temporarily and often with limited decor and no substrate, but can be an ideal choice for using an undergravel filter with some gravel. They can provide adequate filtration for just a few fish or invertebrates and can be used with sponge inserts that come with some under gravel filters. Do not use carbon inserts in quarantine tanks because they will remove any medication added to the water for treating the fish while in quarantine.

Breeding Tanks

If your intended breeding species does not have any specific requirements for substrate when spawning, the lower flow of an undergravel filter may be a good choice. Some fish breed by egg-scattering on the substrate, and using coarse gravel or even marbles above an undergravel filter plate may work well for filtering these breeding tanks. Using the provided sponge cartridges in the filter tubes can prevent fry from going into the tubes.

Small Tanks

Betta tanks and other small aquariums work well using an undergravel filter. They can be used with an air pump and air stone to provide low water flow for slow moving fish, such as bettas. It's also not difficult to vacuum the gravel in small tanks.