Sponge Filters: The Easiest Fish Tank Filter Ever

Sponge filters are one of the most popular filters used in fish stores, fish rooms, and breeding tanks because they’re so reliable and easy to use. But beginners often have many questions on how they work, how to set one up, and how to keep them clean. Check out our step-by-step instructions to help you get started with your first sponge filter.

Sponge filter setup diagram

What is a Sponge Filter?

This most basic of all filters requires at least three components: a sponge filter (which sits inside the tank), air pump (which sits outside the tank), and airline tubing to connect them. The air pump pushes air through the tubing into the hollow cavity inside the sponge filter. Bubbles rise from the inside of the sponge, thus drawing water through the sponge walls. This water suction process mechanically collects debris from the aquarium and gives beneficial bacteria place to grow.

Sponge filters are a long-time favorite of both beginner and veteran fish keepers since they’re cheap, easy to clean, and hard to break since they have very few mechanical parts. Because of the constant bubbling, it provides good water circulation and surface agitation, white being gentle enough to avoid sucking up fish fry, shrimp, and other slow-moving creatures. Plus, during power outages, the beneficial bacteria on the sponge stays in the oxygenated tank water (which gives it a longer chance of surviving), and you can even purchase a battery pack backup that works with our USB air pump in case of emergencies.

For more information on filtration options, read our article on fish tank filters and which one you should get.

Do I Need an Air Stone for Sponge Filters?

An air stone is a small weighted accessory that diffuses the air from your air pump into smaller bubbles in the water. We recommend adding an air stone to the inside of the sponge filter to lessen the bubbling noise and make the filtration more efficient. The air stone creates a steady stream of tiny bubbles (instead of large, intermittent bubbles) that produces constant lift in the sponge filter – much like a continuously running escalator (versus an elevator that starts and stops all the time). 

How to Set Up a Sponge Filter

1. Take apart the sponge filter and remove the plastic strainer from the inside of the foam.

2. Remove the bullseye from the top of the strainer, and put the air stone at the bottom of the strainer. Connect the air stone to the nipple or center of the bullseye using a small length of airline tubing. If the sponge filter is very small, you can simply connect the air stone directly to the bullseye.
3. Snap the bullseye onto the top of the strainer, put the strainer back inside the foam, and then connect the strainer to the weighted base of the sponge filter.
4. Slip the lift tube over one end of the airline tubing roll and connect the airline tubing to the nipple on the top of the bullseye. Then snap the lift tube onto the bullseye.
5. Place the sponge filter into the aquarium and squeeze out any bubbles from the foam if it’s floating.
6. Place the air pump in its final location outside of the tank, and then cut the airline tubing roll (attached to the sponge filter) to the proper length so that it’s long enough to reach the air pump. Connect the newly cut air tubing from the sponge filter to the air pump.
7. If the air pump is located below the top of the aquarium, you need to add a check valve to prevent water from flowing into the airline tubing whenever the air pump is turned off or the power is out. Cut the airline tubing (between the sponge filter and air pump) a few inches outside of the aquarium, and then attach the check valve in between so that the end of the check valve with the flapper (looks like a colored or horizontal bar usually) is facing the air pump. (If you install it backwards, no air will flow when you turn on the air pump, so just flip it around.)

8. Create a drip loop with the power cable of the air pump (to ensure moisture will not make contact with the plug), and then plug in the air pump. Within a few seconds, you should see bubbles coming from your sponge filter.

Why Are Bubbles Coming out of the Side of the Sponge?

There are several reasons why this could be happening, so try checking the following:

• Did you shorten or remove the lift tube? A shorter lift tube does not have as much suction pulling bubbles up the center column, so some air may escape.
• Is the air stone crooked inside the sponge filter? To make it hang straighter, you may need to shorten the tubing attaching the air stone to the bullseye.
• Is the air pressure from the air pump too strong? If a bunch of air is forced into the sponge filter, excess bubbles may leak out the sides.

Which Sponge Filter Do You Recommend?

Sponge filters are a pretty basic piece of equipment, so there’s not a lot of difference between brands. However, after a decade of using tons of sponge filters, we made our own with all the improvements and features that we’ve always wanted. We designed the base and lift tube with a green color to blend in with planted tanks and easily hide green algae growth, whereas the foam sponge is black to best conceal fish waste and detritus that gets sucked in. 

The sponge is made with a coarse foam of 20 ppi medium porosity to easily collect particulate from the water without clogging up too quickly. The surface area is ideal for shrimp and fish to graze on and clean. Plus, the coarse sponge doesn’t trap as much air, allowing it to get nice water flow and sink immediately.  (Fine sponges often have problems with floating, which can cause lack of oxygen in your aquarium and potentially loss of life.)

All of the sponge filters we sell are hollow inside and tall enough so that you can install an air stone inside for more efficient filtration and quieter bubbles. Also, if you remove the lift tube, you can connect another sponge filter on top (without its base) to increase filtration capacity. These sponges can be customized in multiple configurations, since three of the sponge sizes (all except for the nano sponge) can be mix and matched together. The advantage of stacking multiple sponges (versus running them separately) is that they can run off a single air pump line. Then, if you ever need to set up a hospital tank, simply remove one sponge from the stack and it’s already seeded with beneficial bacteria to help the quarantined fish.

How to Clean a Sponge Filter

Yes, a sponge filter helps to clean your aquarium, but it’s essentially like a trash can that collects waste and needs to be emptied out every once in a while. We recommend cleaning your sponge filter once a month or whenever you see a decrease in bubbles (which is caused by the foam getting clogged up with detritus).

1. When taking the sponge filter apart, disconnect the bullseye from the strainer (i.e., take off the whole top part of the filter) so you can easily remove the foam part for cleaning.
2. Use a plastic bag to scoop the foam out of the water so that the detritus won’t spread and make a big mess in the aquarium.
3. Squeeze and wring out the foam several times in old tank water.
4. Reassemble the sponge filter and put it back in the tank.
5. If there are lots of particles floating in the water, just wait an hour or so for the sponge filter to clean it up.

Sponge filters are easy to use, budget-friendly, and very reliable compared to other filter types. If you haven’t tried one yet, check out our line of sponge filters and let us know what you think!