Should a Fish Tank Filter Be Under Water? (Best Practice)

When it comes to aquariums, who doesn’t love colorful, unique fish in a giant water tank with underwater plants and props? It looks lovely, but you must put extra care and effort into making it look that way.

Your tank’s filter is extremely important as it keeps the water clean and helps maintain the balance in the aquarium.

But if you’re just starting up with this hobby, you probably wonder if the filter should be completely submerged or if part of it should be above the water level.

The correct answer depends on the type of filter you are using, but we’ll get a bit more in depth to make it as clear as possible for you.

Should a fish tank filter be fully submerged?

An internal filter must be entirely under water in order to function properly. This way, it will create a water current that moves the water through the filter itself, getting the debris stuck in the sponge and releasing clean water back into the tank.

Internal filters can be placed directly on the bottom of the tank, but they are more effective if you use the provided suction cups to place them on the aquarium’s glass, fully submerged but above the substrate.

internal filter underwater
A basic internal filter is placed under water.

It’s also ideal to try an place them in an area as empty as possible, away from plants with large leaves. While it will still work, the leaves might block some of the particles and make the filter less effective overall.

Where should a filter be placed in a fish tank?

There are various types of aquarium filters you can buy for your tank. The ideal placement of a filter depends on its type.

1. Internal filters are the most common and must be placed entirely under water to work properly. There should be at least an inch (2.5cm) between the top of the filter and the top of the water.

Many internal filters come with an additional aeration or waterfall system. This system (usually a tube that spits the water sucked in by the filter) should be above water for the aeration to work, but the filter itself will still function even if the aeration system is underwater.

But in this case, the aeration system itself makes little sense. And in all reality, you don’t really need one for an aquarium, especially if it has a large amount of plants to provide enough oxygen for the fish.

To sum up, internal filters must always be submerged. If they have an aeration system, that should be above the water – but not the filter (the main unit).

2. Hang on water filters (aka waterfall or cascade filters) are designed to be mounted on top of the aquarium.

It has a tube that goes deep into the tank, sucks in the water which is then filtered and then returned to the aquarium from above the water, hence its name (cascade filter).

These are better suited for smaller tanks, because the water movement won’t be as intense as with an internal filter so much of the debris might remain in the tank.

3. External filters are generally used for larger fish tanks. They have an external unit that filters the water inside the aquarium and minimal elements inside the tank itself.

It usually has two tubes that have to go deep under water: one sucks in the dirty water and takes it to the external unit to be filtered and the other sends back the clean water into the aquarium.

4. Undergravel filters are the least common and, as their name suggests, must be placed under the substrate in an aquarium. I don’t really like or recommend them because maintenance is really difficult.

Things to Keep in Mind

  • Make sure there is always water in the pump chamber to avoid the filter pulling in the air instead
  • Keep an eye on your bubble walls and make sure they are not located directly under the intake tube. Your filter may stop working or even make noise if the bubbles go up the main tube
  • Always be aware of the water level as it should be on the level recommended for your fish tank filter. Remember, most filters require the water level to be around one inch from the lip of the filter
  • Do not use sand; always use gravel for your aquarium.

How to Set Up a Fish Tank Filter

should fish tank filter be under water 1

Once you’ve settled for the type of filter you need for the aquarium, now comes the important part — how to install it. Depending on the type of filter you’re using, the steps will vary slightly, but it’s generally extremely simple to install a filter.

1. Set up and assembly

Start off by assembling and setting up the filter, but don’t plug it in. Follow the steps on your filter’s instruction manual, and it’ll be easy.

Now set up the temperature control system of your tank, and you are good to go for the next step.

2. Gravel and aquascaping

It is recommended that if you aren’t using biological filtration, then two to three inches of gravel would be adequate as it would provide stability.

The minimum estimation is about two pounds of gravel per gallon of water. Make sure to rinse and stir the gravel continuously in a bucket before pouring it into the aquarium.

3. Installing the filter

Now comes the main part; installing your fish tank filter. Once the assembly and graveling are done, place the filter into the tank following the instructions of your particular filter.

For internal filters, put them in the back of the aquarium, using the suction cups to fix it in place.

External filters need only the tubes placed into the aquarium, going deep under water, close to the substrate.

Cascade filters are placed on the side glass of the aquarium, with the tube deep underwater. Make sure to set the flow properly otherwise the water can splash and make a mess around the aquarium.

4. Turn on your filter

After a final check, turn on your filter when the tank is filled with adequate water.

Why Do You Need a Fish Tank Filter?

Just like we humans require clean and hygienic air to breathe, fish also need fresh, clean water to breathe.

One of the most essential parts of fish keeping is the maintenance of the fish tank. This mainly includes proper filtration and cleaning the tank.

So why do we need a fish tank filter? Here are a few facts, especially for beginner fish keepers.

  • The primary purpose of the filter in your aquarium is to remove free-floating particulates, fish waste, dangerous chemicals, excess food particles, and decaying organic matter
  • It is used to prevent ammonia poisoning, which is the poisoning of fish due to the excessive presence of fish waste in the water that they are continually excreting into
  • Filtrations such as biological filtration will remove the presence of harmful bacteria and microorganisms
  • Filtration also prevents cloudy aquarium water that is caused due to the presence of decaying food, floating particulates, and other organic matter

Final Thoughts

Most filters in an aquarium have to be fully submerged or at least have some parts submerged under water in order to work properly. Make sure to always follow the instructions of the particular filter you have to ensure that you install it correctly.

Share the article below:

Leave a Comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.