Why is Your Fish Tank Filter Leaking–How to Fix It

By Eddie Waithaka @aquariawise

Why is Your Fish Tank Filter Leaking–How to Fix It

You come home to find your fish tank water about an inch down and water on your floor!

Your aquarium filter is leaking!

What might be the cause, and how do you fix it?

Keeping in mind your fish will not survive in a tank without a filter!

There could be several reasons why your fish tank filter is leaking.

One possibility is that the filter is not well sealed and is letting water escape. Another is that the filter is not placed flat on the tank (which happens with HOBs). A third possibility is that something is wrong with the unit or one of the parts connecting to the filter.

From experience,…

Canister fish tank filters leak from where you clip the lid to the unit. Check the O-ring (gasket) around the lip for cracks and tear. If the gasket is broken, try to patch it with aquarium-safe adhesive and replace it if the filter is still leaking.

You should find a replacement on sale in your local fish store or online.

That said,…

It is rare for aquarium filter containers and lids to crack and leak unless they have been dropped or knocked around.

In the rest of this blog post, let’s look at why your fish tank filter might be leaking and some ways you can fix it.

Why is Water Leaking from Your Fish Tank Filter

As noted above, fish tank filters mostly leak around where you clip the lid to the container. If you take your leaking filter apart, look for the o-ring, and inspect it, you will likely find cracks and tears.

However, if the gasket (o-ring) is not broken, check that it is well-fitted on the lid grooves. You may also want to inspect (and clean) any dirt and debris on the container and lid slots.

Canister Filter Leaking at The Top

Canister filters leak at the top because of the reason mentioned above.

Your canister filter will leak if you have a broken o-ring (gasket) at the top of the filter between the lid and container. Water will also sip out if the gasket is not fitted into the grooves properly.

If the seal is warped at any point, water will sip through the groove and leak from the filter.

To fix the filter, take it apart to access the o-ring and inspect it for tear. Use the appropriate fish-safe adhesive or epoxy to patch the tear, put the filter back together and test it.

Put food-grade silicon lubricant or vaseline on the seal before you place it back to seal tiny spaces between the gasket and the lid that might allow water to sip out even when the 0-ring is in good shape.

If the filter is still leaking, you may have to purchase a replacement gasket.

You will get a replacement gasket for your filter type in your local fish store or online if the filter brand is trendy in your area or country.

Please note that tears on the seal (o-ring, gasket) can sometimes be hard to find. Your filter gasket may also have multiple tears, and replacing it might be the easier option from the start.

The seal, O-ring, or gasket is the black rubber thing that sits in the grooves on the inner side of your fish tank filter lid. The seal is meant to prevent water from sipping through the slots on which the filter container and lid connect.

Canister Leaking When Turned Off

If you have a canister filter that only leaks when you lose power or turn off the pump for cleaning, back pressure and reverse flow might be the cause.


A canister filter should not leak if the canister and tank differential is within a reasonable limit, but when the pump is off with the canister too low, the pressure in the siphon can get too high for the seal to handle.

If you raise the filter, it should help.

You can also try turning the intake and outflow off and see if the fish tank will stop leaking from the edges.

That said,…

Remember to check for broken or weak seals because they are more likely to leak when the back pressure is too high, as it would in this instance.


Ensure that a clogged impeller or hoses or a faulty motor are not causing your filter to overflow and leak.

Hang on Back Filter is Leaking (Why)

Hang-on-back filters are attached to the back of the aquarium. The water is pumped through the filter and back into the tank. These filters are popular because they are easy to use and are good at keeping small fish tanks clean.

However, one of the drawbacks of hang-on-back filters is that they can leak.

Most hang-on-back filters will leak if not placed flat on the tank. For instance, if the little plastic leveler falls out, the filter will tip and lean backward, causing it to leak.

Your HOB filter may also leak around the o-ring if the seal is cracked or dry or if the part where it sits fills with poop and detritus.

Apply vaseline on the O-ring if it’s dry to stop the leak. If the seal is cracked, replace it with a new one.

If your HOB filter is level and the seal is not broken or displaced, a dirty filter sponge may be causing the leak.

When the filter sponge gets full of gunk, the flow of water becomes weaker to the point where it pushes the foam block upwards and causes the cover to come off, and then leakage happens shortly after.


Unless the leaking is around the O-ring, ensure the filter media is not loaded and pushing from the water flow, raising the water level on the top filter case and overflowing.

That’s all for this post.

We hope this information was helpful and that your filter is no longer leaking.

See you in the next one.

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