Should a Fish Tank Filter be Fully Submerged?

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Countless times I’ve been wondering where I should position my aquarium filter. The more that I learned about the different filter types, the more I got confused. To make sure I position the filter’s body, inlet, and outlet correctly, I investigated the topic quite profoundly. Now, I am willing to save you some trouble and share what I’ve learned.

Air-powered, Undergravel, and internal filters should be fully submerged. That allows water to flow in an efficient closed cycle. However, hang on the back or canister filters are not submersible and typically positioned outside the tank. Also, their outlet should be placed two inches above the waterline.

As we move forward, I will teach you precisely where each filter type should sit in your aquarium. I will also address heaters’ cases and explain which devices should be entirely submerged in the water.

Should a Fish Tank Filter be Fully Submerged?

Filters are generally shipped in pieces, and you have to assemble them when they arrive. While manufacturers provide clear guidelines on installing their products, they usually assume that you know a little about filters, particularly regarding where they should be positioned.

Every aquarist understands the critical role an aquarium filter plays. It removes contaminants from the water. But where does it perform this role from? Should it hang somewhere on the exterior walls, or can you submerge it fully?

The answer to both questions is ‘Yes. You can submerge the filter, hide it under the gravel, place it beneath the tank, and hang it on the exterior walls. Obviously, you cannot do all this with a single filter. The position will vary depending on the type of filter:

1. Air-Powered Filters

Some people call air-powered filters’ Interior Filters’. But that term is quite broad. You can apply it to various types of filters, which is why it doesn’t make sense to call Air-powered filters’ Interior Filters’. They are not the only interior filter on the market.

Air-powered filters are primarily found in small tanks. They are too weak to contend with the tanks that are typically paired with HOB filters. Boasting a sponge material and typically powered by an air pump, air-powered filters should be fully submerged in the water.

2. HOB (Hang-on-Back) Filters

As their name suggests, these filters are found on the back of the tank. You can install them on tanks of all sizes. Naturally, you cannot submerge them. HOB filters take up a lot of space at the back of the tank. 

Some aquarists dislike them because they prevent you from pushing the tank against a wall. As you might have guessed, HOB filters are not submersible. You typically hang them in a way they catch the back wall of the aquarium, while the outlet is two inches above the waterline. That allows gravity to carry the water back into the aquarium once it has been filtered.

3. Canister Filters

These filters are suitable for medium and large tanks. Comparable to HOB filters in size and function, you may find canister filters in cabinets under the aquarium. Obviously, they are not submersible and are kept outside the tank.

That is one of the advances of that particular filter. It does its job while keeping the tank’s appearance minimalistic and clean. While you can potentially hang canister filters at the back of the tank, they are better off separated. 

Even though canister filters are generally placed under the aquarium, their output is typically found two or so inches below the water level. Yet, that isn’t mandatory. You can always position the outlet above the water level. But this will produce a lot more splashing than you might want.

Internal filters seem pretty attractive because they are contained within the tank. However, you should know that they are best suited for small tanks. Larger tanks typically require external filters. That said, people love submersible filters because they provide a lot of agitation.

4. Undergravel Filters

As their name might have already revealed, under gravel filters are placed under the substrate.[1] They have a perforated plate covered by gravel, some uplift tubes, and an air pump. As you might have guessed, these filters must be fully submerged to do their work.

Undergravel filters should sit below the gravel, specifically the perforated plate, not the uplift tubes or the air pump.[2] The air pump is actually closer to the top of the tank. That prevents the water from flowing back in the event of a malfunction.

5. Internal Filters

While Air-Powered and Undergravel filters are technically internal filters, when people talk about ‘internal’ filters, it is usually about a device similar to a HOB filter except for the fact that it is placed inside the tank.

The height of an internal filter doesn’t matter so long as it is below the water level. The filter should be fully submerged. If suction cups have been provided, you can attach it to the wall below the water level. Otherwise, please leave it in the corner.[3]

In short, you also have wet/dry and fluidized bed filters to consider. Though, they are primarily found in the homes of experienced aquarists. Since they are not that prevalent, I will say that neither option is submersible.

Where Should a Filter be Placed in a Fish Tank?

Here is a table that summarizes where should aquarium filters be placed in fish tanks:

Filter typeCorrect placement
Under gravel filtersShould be placed under the substrate
HOB FiltersShould hang on the back of the aquarium
Canister FiltersAre usually placed in cabinets under the aquarium
Fluidized Bed FiltersAre hung on the back of the aquarium
Wet/Dry FiltersAre generally situated beneath the aquarium

But with internal filters (including air-powered filters), you have two options. The first would be hanging these filters on the interior walls of the aquarium using suction cups.[4] However, the most common approach is to place the filters on the aquarium floor, preferably in the corner.

Can You Fully Submerge a Fish Tank Heater?

You can fully submerge a fish tank heater, as long as it is categorized as submersible, substrate, or an in-filter heater. Hanging heaters, on the other hand, should not be entirely submerged. Instead, they hang above the waterline, with merely the glass-covered part being underwater.

Every aquarium requires a heater to keep the temperature within the appropriate range. However, because heaters are such delicate devices designed to generate heat, the notion of submerging them worries people. 

But as with aquarium filters, the position of a heater will depend on the type of heater:

1. Hanging Heaters

Hanging heaters are the most common type. You will find them in most aquarium kits. As their name suggests, you are supposed to hang them from the edge of the tank. In other words, they are not submersible. However, they do have a heating element that enters the water.

That part is usually covered by a glass tube, which makes it waterproof. Hanging heaters are a little risky because you can accidentally knock them off. Additionally, if the glass tube breaks, the heater could electrocute your fish.

You rarely see them on marine tanks because the salt tends to cause corrosion. If you want to buy a hanging heater, you should know that your aquarium cover needs an opening that can fit the heater’s head. Some covers are designed with a section that can be cut away.

2. Submersible Heaters

Submersible heaters are popular among experienced aquarists. As their name suggests, they can be fully submerged in water.[5] They are superior to hanging heaters because they are out of reach. You don’t have to worry about knocking them down.

Submersible heaters are also flexible. That is to say; you can place them in any position that suits you, be it horizontal or vertical.[6] You can experiment with different angles until you identify a position that fits your aquatic environment’s needs.

Some submersible heaters come with suction cups that you can use to attach the devices to the wall. But in many cases, it is easier and more efficient to place the heater horizontally at the bottom. That allows the heater to warm the water uniformly.

You can position multiple heaters in different corners of the tank to warm massive aquariums. Your primary objective where these tools are concerned is to ensure that they are fully submerged before you turn them on. Keep this in mind when you perform water changes. 

Submersible heaters are superior to their exterior counterparts because you can perform water changes without removing or switching them off. However, this only applies to water changes, ensuring that the heater remains below the water level. 

In case the heater continues to operate while above water, the coil will heat the tube to unsafe levels and might crack as a result. It is worth noting that some submersible heaters are placed inside a trickle filter’s sump to keep them safe from violent fish that might attack and damage them.

3. Substrate Heaters

Like under gravel Filters, substrate heaters are fully submersible. They have a coil in an insulator that you insert beneath the substrate. The coil (or wire grid) will heat the gravel. In turn, the gravel will distribute the heat through the water, raising the temperature.

The thermostat will ensure that the heater switches off once the water reaches the required temperature. Substrate heaters are not that common. However, they are perfect for people with a lot of foliage who hate that the substrate keeps lowering plants’ roots temperature.

4. In-Filter Heaters

It might surprise some people to learn that some filters have in-built heaters. They will heat the water as they filter it, saving you the trouble of buying individual heaters and filters. The position of the heater inside or outside the tank will depend on the position of the filter.

While submersible heaters give aquarists a lot of freedom, you are still expected to employ some tact when positioning your heater. For instance:

  • Aquarists are typically encouraged to keep their heaters near the filter discharge to maximize heat distribution.
  • You should keep submersible heaters away from objects such as decorations. If your fish is caught between a heater and an object, it may either suffer burns or damage the heater to escape.
  • The glass sheath of a heater shouldn’t come into direct contact with objects such as the aquarium walls, the decorations, or even the substrate. That could shatter the glass sheath.
  • New heaters should be left in the aquarium for at least thirty minutes before they are plugged in and switched on. Once you unplug a heater, you should leave it in the water for this same duration before taking it out. In both cases, the objective is to prevent the glass from breaking due to a drastic temperature change.


Aquarium filters are usually fully submerged. That includes internal, under gravel, and air-powered filters. For those filters, it is possible to include the outlet underwater. The filtration will continue, and you will have less annoying bubbles.

However, bear in mind that placing the outlet two inches above the waterline is ideal since it also helps with oxygenation. That is also the case with HOB and canister filters. They are usually placed outside the tank and not submergible.

There is a similar approach to aquarium heaters. Hanging ones are placed above the waterline, with only the glass-covered part being submerged. There are also submersible heaters that are entirely underwater and usually placed horizontally.