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Copepod Vs Amphipod – 5 Obvious Differences Between The Two!

With the recent surge of new people to the fish keeping hobby, especially marine tank keeping, there have been a large number of questions month in, month out about the various random things that can end up living in our tanks from the community.

One of the most common things that we see people reaching out with questions about is copepods due to many beginners not knowing what they are and often worrying that the copepod are taking over their tank if there is a copepod bloom.

We already have the following articles on questions we have seen people asking about copepod in their tank:-

Since publishing them though, we have noticed a number of people reaching out and asking for a copepod vs amphipod comparison article to try and help them better identify if they have copepods or amphipods in their tank.

Please keep in mind that many marine tank setups will have both copepods and amphipods in their tanks rather than just one of these little critters.

Although they are unsightly, both copepods and amphipods can actually end up helping with a wide range of common problems that we see people have with their tanks due to how much detretus, left over food, and in some cases, algae both of these critters will ask.

Still, we wanted to publish this article going over the differences between copepods and amphipods to try and help our readers tell the difference between the two.

What Is The Difference Between Copepods And Amphipods?

Our list below goes over the most obvious differences between copepods and amphipods and we will cover them in more detail below:-

  • Size Differences!
  • Color!
  • Shape!
  • Appendages!
  • Behavior!

You will commonly find additional differences with some of the larger types of amphipods too but we will be focusing on the differences that apply to all amphipods for this article to help our readers tell the differences between the two.

What Do Copepods Look Like?

The above is a heavily magnified image of a copepod but they will just look like small white dots to the human eye. Due to there being so many different types of copepod, there can be a number of differences between the various types of copepod.

What Do Amphipods Look Like?

The photograph above is a magnified image of an amphipod but there really are a large number of amphipods that look very different to each other. With there being around 10,000 types of amphipod, there is no way that we can show what they all look like.

Size Differences!

The most obvious difference between copepods and amphipods is the size differences between the two.

Copepods will usually be between 1mm-2mm in length when fully grown while mosttypes of amphipods will be considerably larger with some types of amphipods being able to grow to around one inch in length but this is rare.

There are around 10,000 different types of amphipod and the majority of them will usually grow to a maximum of 10mm in length making them between ten and five times larger than the average copepod.

It is estimated that there are between 2500 and 3000 different types of copepod and almost all of them max out at a lenght of between one and two milermeters.

Just keep in mind that baby amphipods will be small and may be mistook for a copepod but they will grow quickly and make the differences between them obvious.


Another common difference between copepods and amphipods is their color.

The vast majority of copepods are either white or a pale yellow or brown color but aphipods have a much wider color range.

Although many amphipods are also white or pale colors, there are a large number of amphipod types that are darker colors such as red, dark brown, and orange.

You can usually combine the color differences with the size difference to workout if you have copepods or amphipods in your tank as most of the smaller types of amphiepods tend to be darker colors so it rules them out as being copepods even though they are a similar size.


This next one can be much harder to tell due to many types of amphipods being a similar, round shape to copepods but some amphipods have some veriety in their shape.

If the creature in your tank is long and skinny or kind of looks like a grasshopper then there is a higher chance of it being an amphipod rather than a copepod.

Although there are some copepods that can have variations in their shape, their much smaller size tends to make this difficult to see with your eye and unless you have some sort of magnification, most copepods will look the same when it comes to their shape.


Amphipods tend to have far more appendages than copepods and although this is not always true, it usually is and the smaller size of copepods usually makes it difficult to workout the size of the copepods in most tanks.

The younger amphipods or the smaller types of amphipods will usually still have more appendages but their smaller size will often make it difficult to see them.

Thankfully, many of the smaller types of amphipod in marine aquarium will be a darker color than their similar sized copepods helping you identify them as amphipods.


The behavior of copepods and amphipods can be very different where as copepods will usually be feeding or settle on your aquarium glass, amphipods will usually be much more active in the tank when moving around.

This is not always true and specific type of amphipod that you keep in your tank will definatley come into play.

Are Copepods And Amphipods Good For A Reef Tank?

Now that we have looked at some of the key differences between these two common creatures, you might be wondering if they are good for a reef aquarium and the answer is yes!

Both copepods and amphipods can provide a range of benefits to your reef tank such as helping to control algae growth, providing a source of food for larger fish and invertebrates, and even help to improve water quality.

In most reef tanks, amphipods and copepods will breed quickly and become a sustainable food source for many of the inhabitants in your aquarium with some fish keepers intentionally adding them to their tanks.

If you have a reef aquarium then it is definatley worth considering adding some amphipods or copepods to your tank as they can provide a range of benefits and help to create a more balanced ecosystem.

Just keep in mind that once you have added them to your tank, they can be very difficult to remove and some people really dont like the look of copepods or amphipods in their tanks.