Can You Over Filter a Fish Tank?

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There is a lot of discussion about providing fish tanks with the right level of filtration. Most of the time, people talk about the consequences of under filtering a fish tank. But what about over filtering? In this post, I want to take a look at the different ideas relating to over filtering a fish tank and some things you need to take into consideration.

As long as the flow caused by filters isn’t too powerful for your fish, then over filtering shouldn’t cause them any harm. You are much better off providing your tank with slightly too much filtration rather than not enough.

Too Much Flow Can Be a Problem

The biggest problem you’re going to have with too much filtration in your fish tank is flow. Filters can generate quite a powerful current and your fish don’t want to live in a washing machine. Some fish are fine with a gentle current and actually quite like it. Certain fish – like betta – prefer much calmer water. Do some research on your specific fish to get an idea of the level of water movement they are comfortable with. An effective way of reducing flow is by using foam pads to baffle the filter.

Bacteria & Contact Time with the Water

Biological filtering is extremely important. This is when beneficial bacteria breaks down ammonia into nitrites, and then into much less toxic nitrates. There is an argument that over filtering will reduce the contact the bacteria has with your water.

Beneficial bacteria will grow all over your fish tank, but your filter provides the perfect place for it to grow. As your filter pulls water through it, the water has contact with the bacteria. If you have too powerful a filter, the water will move through so quickly that it doesn’t have a lot of contact time with the bacteria. And if you have a filter that is physically large, the bacteria will be spread out within the filter, essentially diluting the bacteria and making it less effective.

Although this logic appears to make sense, it isn’t clear how much of an impact it has and if it really makes that much of a difference. Overall, I would be more concerned with the flow of the water than with the bacteria’s contact time with the water, but it is worth keeping in mind.

Multiple Filters Are a Good Idea

As long as you keep the flow of the water under control, it can be a good idea to have multiple smaller filters in your fish tank rather than one large one. If one of the filters stops working, you will still have some filtration in your tank until you get a replacement. And if you plan on setting up a new tank, it also means you can get the new tank started quicker with the friendly bacteria by using one of the old filters in the new tank.

You Still Need to Change Your Water

Some people think that by providing your tank with lots of filtration, you don’t have to do as many water changes, or any at all. This is not true! Biological filtering breaks down toxic ammonia into nitrites, and then into less toxic nitrates. The keyword here being less toxic, not non-toxic. High levels of nitrates are still dangerous to your fish, so you still need to perform water changes to get rid of nitrates, no matter how much filtration your tank has.

How often you should change your fish tank water depends on a few factors, but a good guide is 25% once per week.

How Much Filtration Should I Have?

Most filters have a recommended tank size printed on them. Make sure you choose a filter rated for at least as big as your tank, but preferable a bit bigger. If you plan on using two filters, each one can be a bit smaller, but make sure they both add up to at least the size of your tank, but again, preferably a bit bigger. So if you have a 60 gallon tank, two filters rated for 40 gallons would be ideal.


So to answer the question, “can you over filter a fish tank”, I would say that as long as the flow isn’t too strong for your fish, then you aren’t going to do them any harm. Over filtering might lose some efficiency because the beneficial bacteria won’t have as much contact with the water, and there will come a point where you will be wasting money and electricity on too powerful a filter. However, you are better off going slightly overboard with your filters rather than under filtering. As long as you stick to the tank size guides on the filters, you should be good.

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