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Can an Aquarium Be Over-Filtered? The Risks and Effects

Fish aquariums are prone to bacterial growth and algae blooms, which is why many fish tank owners resort to using two filters. Twice the filtration equates to a much clearer, healthier aquarium, right? Unfortunately, it’s not always that easy. It’s important that you have the proper flow rate for the fish, plants, rocks, and structures in the tank.

Quick Answer

An aquarium can be over-filtered because large filters have large pumps that can move the water too quickly. When the water is cycled at an excessive rate, spread sediment all over the tank, push the fish around, and more. The water should pass through the filter four times an hour.

Throughout this post, I’ll discuss how you can choose the proper filter size for your tank, why some aquariums require more filtration than others, and common signs that you’re filtering too much (or not enough). I’ll also show you when you might want to use two filters instead of one. Enjoy!

Over-Filtering an Aquarium

You can over-filter an aquarium in many ways. Whether you have too much flow from a high-powered pump or there’s too much debris caught in the filter, choosing the correct size will bring the tank back to good condition. However, there are a few things you should look out for to know if it’s over-filtered.

Here’s what you should know about potential over-filtration:

  • Over-filtering an aquarium can disturb the rocks and plants. When the water swooshes around the tank, it’ll pull the sand or other substrate out of the ground. The water will look cloudy, and the plants won’t settle. You might notice the plants are always on their sides or swaying in one direction.
  • Too much filtration can make it difficult to see everything in the tank. Whether you’re worried about bubbly water or turning everything too often, there are several ways over-filtration can make everything harder to see. In fact, it could throw the decorations around and quite a display of chaos in the tank.
  • The tank can’t build a proper layer of healthy bacteria if the water’s moving too quickly. Fish tank filters need to cycle themselves by building bacteria. Once they have a good amount of bacteria, they can balance the water and prevent excess algae growth. If the water’s moving too fast, the bacteria can’t settle.
  • Some fish require more filtration than others. Cichlids are some of the messiest aquarium fish. According to Aquarium Store Depot, cichlids are incredibly dirty when they’re eating and pooping. If you have one of these beautiful fish, you might need more filtration than usual.

As you can see, over-filtering a fish tank has a host of drawbacks. There are many ways to filter your fish tank, so it’s best not to assume the only culprit is the pump.

The two primary methods that clean your aquarium are biological and mechanical filtration. Let’s discuss the options you’ll have below.

Biological vs. Mechanical Filtration

Mechanical filtration is the most common type. It includes filter socks, sponges, and floss pads. These filters are incredibly effective at removing contaminants in the water, including algae, fish poop, and other debris. However, they perform best when paired with natural, biological filters.

Although they’re called biological filters, these organisms come in many shapes and sizes. They include certain fish that eat the algae and poop at the bottom of the tank, plants the regulate the carbon and nitrogen in the aquarium, and bacteria that promote a healthy living environment.

Combining biological and mechanical filters is a great way to have a clear tank, but if you had to choose one, mechanical filters are the best option. They’re easy to use and remove debris much quicker.

Can a Filter Be Too Strong for a Fish Tank?

A filter can be too strong for a fish tank if you have sensitive fish, such as betas. Gouramis need to lay their eggs in bubbles on the water’s surface, so a high-pressure filter will create a strong current, preventing them from breeding. Many fish can be stressed by a strong filter because the water is moving faster than it should be.

Review this five-step process to know if your filter is too strong:

  1. Some fish can’t handle a strong current, which could make it an uncomfortable or hazardous environment for them. If your fish is always swimming too low to the ground or won’t move from a hiding spot, it could be because the filter is too strong. They can’t relax if it’s going too quickly.
  2. If you notice sediment or plants swirling around the tank, the filter is likely too strong. A filter that moves the sediment, plants, or fish is far too powerful for your aquarium. These issues are telltale signs that it’s time to replace it with a lower PSI pump and filter system to calm the water.
  3. Gourami fish breed and lay eggs in a bubble on the surface, but a strong filter can break the bubble or make it impossible for the fish to lay eggs. The Aquarium Advisor explains that gouramis need calm water to breed. If the water is rippling from edge to edge, they won’t have any room to lay eggs.
  4. Decorations, plants, and fish can get stuck in the filter. If you’re always removing plants and other items from the filter, it might be too strong. However, this common problem could also mean you’re placing everything too close to the filter. If they’re uprooted, the filter is likely too strong for the aquarium.
  5. Your fish can become stressed from the constantly excessive filtration. Are your fish laying low, not moving, or stay on the far side of the tank from the filter? These could be signs that they’re stressed or anxious. Yes, fish are capable of being stressed, just like humans and all other animals. Long-term anxiety in a fish can cause many health concerns.

Knowing what to look for in an over-filtered tank is half of the battle. If none of these issues are present, you’ve likely chosen a filter that’s the proper size. However, some people prefer using two filters to keep the water moving and prevent dead zones. If you’re interested in this technique, head to the next section.

Is It Okay to Have 2 Filters in an Aquarium?

It’s okay to have two filters in an aquarium if it doesn’t disturb the fish. Having an extra filter can build its bacterial cycle, preparing it for another tank. Messy fish species need more filtration, so a spare filter could make a world of difference.

That being said, there are many downsides to having two filters. They could be harmful to the tank, so review the pros and cons below to make an educated decision.

Pros of Having Two Filters

  • You’ll always have a backup filter in case one of them breaks. Perhaps the most common reason people have two filters is they don’t want to head to the store or order one online if the other breaks. Having a backup filter can prove to be quite beneficial as long as it doesn’t over-filter the aquarium.
  • One filter can be prepared and cycled for bacterial growth, allowing you to set up another aquarium. The Spruce Pets details how helpful a layer of healthy bacteria can be for a fish tank. This layer typically takes a few months to develop, so you could have both filters running and use the other for another tank when it’s cycled.
  • Some fish need more filtration, as mentioned earlier in the post. If you have cichlids, goldfish, or other messy fish, an extra filter might be the best thing for your aquarium. You won’t have to worry about manually cleaning the tank as much, but it’s not ideal for fish that don’t cause too much of a mess.
  • An aquarium without enough biological filtration and cleaning needs more filtration. If you don’t have any of the previously mentioned natural filters (bottom feeders, plants, etc.), you might need another line of filtration. Those who follow this method can use a slightly weaker secondary pump to add bonus filtering.
  • If you have a weak filter, another filter of the same size could potentially help the process. Place a filter on both sides of the aquarium or side by side. Either method will boost the filtration. However, it’s crucial to ensure the water isn’t moving too much by looking for sediment movement, uprooted plants, or stressed fish.

Cons of Having Two Filters

  • Over-filtering an aquarium will lead to the previously mentioned downsides. Two filters can double the pump’s pressure and disturb the water. There’s no reason to double down if you don’t have messy fish or a low-quality filter. Not to mention it’s more affordable to get one filter instead of two!
  • A second filter often takes up too much space, making the aquarium look crowded. Your fish could be upset by the lack of swimming room, while you might be let down by the tacky appearance of too many mechanical parts in the water. This look can make it seem less natural and eye-catching.
  • Two filters typically lead to twice the work. While you’ll remove more algae, you’ll have to clean both of the filters. Not only that, but when it’s time to replace a filter cartridge or sponge, you’ll have to replace both of them. Keep spares on board if you have space to store them.
  • Snails usually stay away from filtration systems, which means they won’t clean the glass and rocks near both of them. Your snails shouldn’t be blown off of the glass if they get too close to the filter. Too much pressure along the walls will discourage them from cleaning everything, making them a less effective biological filter.

How Many Times Per Hour Should You Filter Your Aquarium?

You should filter your aquarium about four times per hour. This means that a 32-gallon tank should have a filter that moves at a rate of at least 128 GPH (gallons per hour). This rule of thumb makes it easy to choose the right filter size, but you’ll need up to six cycles per hour if you have messy fish.

Keep these four points in mind:

  1. Most filtration systems should cycle the whole tank four times per hour that they’re on. The Aquarium Guide clearly states every aquarium should move the water through the filtration system four times hourly to prevent algae buildup without over-filtering it. Anything less than three times won’t be effective enough.
  2. Consider a variable speed fish tank pump and filter combo. Variable speed pumps let you control how fast the water moves. You can adjust the pump to go faster or slower, depending on the fish in the aquarium and how dirty it looks. Most VPSs are much more energy-efficient than traditional pumps, too.
  3. Heavily decorated aquariums might need an additional small filter to prevent dead water zones. Dead zones are places where the water is still and unfiltered. They’re prone to growing algae and cloudiness. Having a lot of fixtures in the tank could make it difficult for a low-GPH filter to get the job done. Consider biological filters, too!
  4. Always monitor your fish’s behavior and how often the filter needs to be cleaned to know if you chose the right size. If your fish seem happy and energetic and the water looks clear, you’ve achieved a healthy, natural balance. You can increase or decrease the filtration to accommodate potential issues.

Note: Messier fish will likely need more filtration than the rule of thumb mentioned above. Pisces Pets recommends cycling 15% freshwater twice weekly to prevent the water from getting too dirty. Additionally, you could bump up the four cycles per hour to six or seven times per hour to promote a clean, healthy swimming environment for the fish and plants in the tank.


Now that you know how to choose the proper filter size for your aquarium, you don’t have to worry about over-filtering the tank. Your fish and plants will thank you, and the aquarium will look much healthier and more vibrant.