Diatoms Vs Dinoflagellates (Reef Tank) Identification and Treatment

Image Credit: u/Zigrivers via Reddit
Image Credit: u/Zigrivers via Reddit

Diatoms Vs Dinoflagellates (Reef Tank) Identification and Treatment


A common problem in aquarist’s reef aquariums is the presence of brownish algae coating everything from the sand to the live rock to the glass. It can look unpleasant and outright disgusting if it is allowed to thrive but did you know that not all brown alga is created equal? Specifically diatom algae vs dinoflagellates.

The easiest way to tell the difference between diatoms and dinoflagellates is to disturb them. If they clump together they are dinoflagellates. If they disperse like sand they’re diatoms.

Knowing the difference between diatoms and dinoflagellates is crucial in knowing how to handle an outbreak of either species. Learning to identify the difference between species using the paper towel technique and treating the algae types individually is important to keeping your aquarium healthy.

Why is the sand in my saltwater tank turning brown?

That brown substance isn’t waste despite what you might think. It is more likely than not wither diatoms or Dinoflagellates spreading throughout your reef aquarium and knowing the difference should affect how quickly you deal with the nuisance and the method that you choose to do so.

Both species have different reasons for their presence within an aquarium but will spread across sand, glass, and live rock to thrive.

Are Dinoflagellates Diatoms?

Though the two might seem similar to one another at first glance, the two species are actually quite different.

Diatoms are a type of algae that thrives off of excess phosphates, nitrates, and silicates in your water.

Dinoflagellates, on the other hand, thrive in nearly the opposite conditions. They can begin to overpopulated a tank that has sterile-like conditions.

So, say you thought the problem was diatoms and chose to perform a water change to help deal with the water parameters. Well, if it turned out to be dinos, you just helped to make the aquarium more sterile and helpful for the growth of their population in your system.

What is the difference between diatoms and dinoflagellates?

There are several differences between the two species. Even though they both just look like gross algae in appearance, they have radically different needs and can have a huge difference on your reef aquarium and its inhabitants.

Dinos thrive off of sterile environments and can produce toxins that are bad for different members of your aquarium. Your fish, coral, and snails can all be in danger with a dino population that is out of control

Diatoms meanwhile require more nutrients in the form of the presence of phosphate and nitrate to thrive as well as silica. Diatoms are not toxic but can throw off the parameters of your aquarium causing an issue for the entire ecosystem if not properly handled.

As bad as both seem, in low numbers, they can both provide benefits to your reef aquarium.


Dinoflagellates are a type of protist. What that means is that they technically can act like a plant and an animal; however, with our eyesight, we normally just see them as something more akin to algae. They use photosynthesis to grow and they can eat protozoa. There are hundreds of different kinds of Dinoflagellates but many are similar to one another.

Are Dinoflagellates good or bad?

Let’s be very clear. You want good Dinaoflagellates to be present in your tank in a moderate population as they can help fulfill one of the first rungs in the food chain. However, if aquarium parameters aren’t ideal, the once helpful species can quickly become nuisance algae.

On the other hand, some species are just nuisance algae that spread too quickly to be any benefit to an aquarist.

Can dinoflagellates kill fish?

Yes, they can and will especially if they are the highly toxic Ostreopsis.

Can dinoflagellates kill coral?

Yes, dinos can kill coral, unfortunately. This is another reason why it’s imperative to recognize the signs of a dino infestation and quickly act to get it under control.

How to identify Dinoflagellates

The best way to identify Dinoflagellates (dinos) in your tank is to siphon a sample out of your tank into a small see-through container, preferably with a lid. For an easier sample, you can always use a turkey baster to try and grab a few specimens. Shake the container and finish by filtering the water through a paper towel. Once the water is seemingly clear again, leave it alone for about an hour and if it is a type of dino, then evidence of dino will begin to appear again.

Once you know that it’s a dino, you can begin to investigate its physical characteristics to determine what specific type it is.

You know that it is becoming a problem based on the appearance of your tank. Look for brown-looking lime in high-flow areas or areas that are well lit. Are some of the inhabitants of your tank dying? How are your snails and other invertebrates? Check your pH and if it is low for seemingly no reason, there is a potential that dinos could be the cause.

Types of dinoflagellates

The type of dino you have in your reef tank can make a huge difference in your reaction to it. Knowing which species is present should dictate how quickly you start to resolve the problem like Ostreopsis. These four common types of Dinos might appear in your tank and being familiar with.

They all have slightly different appearances and they all share some characteristics so don’t let one species confuse you for another. These four dinos all produce air bubbles in the tank while also trapping some of those air bubbles between their slimy body and the glass of the tank. This can be a useful way to narrow it down that the brown algae-like substance you are seeing are dinos.

  • Ostreopsis Dinoflagellates – The Ostreopsis is one of, if not the worst dino you can find in your tank. This is one of the largest dinos which is part of the reason their spread is harder to control as it prevents other microscopic organisms from being able to adequately prey on them. They reproduce quickly too which is why it is important to try to handle them sooner rather than later as the longer you wait, the more troublesome they will become for you. Worst of all, Ostreopsis is toxic so organisms that manage to eat it or confuse it with healthy algae and can die.
  • Prorocentrum Dinoflagellates – Prorocentrum is lighter brown and can have giant blooms. Prorocentrum looks really ugly in your tank and can produce a toxin that can be harmful to snails and other organisms searching for algae. This pest can be found somewhere near the middle and upper levels of your tank and will stick to any surface it can find.
  • Amphidinium Dinoflagellates – Amphidinium is also a largely harmless kind of dinos but that doesn’t mean you shouldn’t be wary of large amounts of them. These dinos have a darker brown color and can produce a few toxins that can be harmful to snails. The biggest issue with these dinos is that they prefer your sandbed which means that accessing them to treat them is tricky. Often, it might be better to leave these dinos alone since most treatment methods could be more harmful to other tank inhabitants but it depends on the context of the situation.
  • Coolia Dinoflagellates – Coolia is anything but cool in your tank as they are related to Ostreopsis. However, this species is more likely to be found at the bottom of your tank on the sand bed. They can produce a decent amount of toxins that can be harmful to your snails and fish. They are a dark brown color.

What are Dinoflagellates?

Dinos are a type of single-celled organisms that comes in several species that can accumulate in your reef tank. Though low amounts of dinos are to be expected in a healthy tank, the problem becomes when highly toxic types are present or the total population is out of control.

Are dinoflagellates algae?

Technically, they aren’t conclusively algae but many people call them algae anyway due to their brown appearance. Since most aquarists don’t want brown algae in their tanks, lumping both groups together under the umbrella term as it’s mostly used to quickly reference that there is some brown plant-like material in your tank you are trying to rid yourself of.

What causes dinoflagellates?

Like we have briefly discussed till now, dinos are a good thing to have in a healthy tank as they are one of the first pieces of the food chain. The problem is when they aren’t kept in check properly and get out of control, which happens when the tank is nearly sterile. A sterile tank has few nutrients present in the tank, so few to no fish, little food intake, etc.

What do dinoflagellates eat?

Dinoflagellates will eat a wide assortment of organisms including other dinos. They will eat diatoms and some will eat larger species like algae or zooplankton. They can become a huge disruption to your tank’s ecosystem if they are allowed to get too far out of hand.

Will dinoflagellates go away on their own?

As long as light and food sources are abundant for the dinos, they will be doing their best to stick around as long as possible. Chances are you are going to need to change something about your routine or manually remove the pests to get rid of your dinos.

How to get rid of dinoflagellates

If you do have a take where the dino population is out of control, it will be important to consider a few things before you try to eradicate them. Different methods will be more effective for different outbreaks and some may pose problems for your tank’s inhabitants if you aren’t careful.

The first thing you want to do is try to manually remove as much of the algae mucus substance as possible. The best way to do this is with a filter sock which will catch the dinos easily. While you are trying to deal with the dinos in your tank, you might consider introducing carbon which will help to negate the effects of the toxins that the dinos give off.

You don’t want to simply change the water of your tank or other common cleaning methods that you might try for other species of algae or water parameter problems. Remember that dinos love environments that have a lack of nutrients and nothing is better than a completely clean tank for them. This is why we want to take care of a few things before we start changing water and other care routine practices.

Increasing nitrates and phosphates is another method but be warned that with high levels of nitrates and phosphates other organisms and parameters can be affected. make sure you are not risking anything by following this method.

You can try a day blackout curtain or turning the lights off for around 72 hours. This will help to prevent photosynthesis which should keep the dinos from reproducing. Using the lights is a great way to help stop the dinos in their tracks but it will not remove them entirely. you will need to use other methods to remove all dinos from your aquarium.

  • Will Vibrant kill Dinoflagellates? – Vibrant will most certainly take care of the dino problem but be wary as it can have a ripple effect on your tank. In some cases, it could even lead to a cyano outbreak in your tank so you are trading one problem for another.
  • Will Phosguard remove Dinoflagellates? – Phosguard can certainly help but it is at its most beneficial when you are using it in conjunction with other methods of removal.
  • Dosing hydrogen peroxide for Dinoflagellates? – Hydrogen peroxide can help to eradicate dinos but be careful not to use too much in your reef tank. The rule is to use about 1 ml for every 10 gallons of water.
  • UV sterilizer for Dinoflagellates? – A UV sterilizer can be an effective way to deal with dinos especially during the blackout step of your removal. You should run it during the entire blackout period if your can.
  • What eats Dinoflagellates?
  • Copepods can eat dinos if you haven’t already introduced some into your tank. Copepods are a great way to keep them in check in the first place and if you need to add more they can help in the removal of the dinos over time. Certain cleanup crew members will also help with the removal of the dinos but remember that some species are toxic and can pose a risk to your clean-up crew.
  • Do copepods eat dinoflagellates? – Yes, they do so a good thing to do is to order a bottle of copepods and introduce them all to the areas of your tank that are infested with dinos. Don’t worry about adding too many as they will die off to an appropriate level as the dinos and other nutrients begin to disappear.
  • What cleanup crew eats dinoflagellates? – A lot of snails will eat the dinos but depending on the species, the toxins could also pose a risk to the snails. It is better to deal with the dinos in other ways but it is good to know what organisms you should be trying to protect. Here are some great examples that will eat dinos.
    • Nassarius snail
    • Rounded Turbo snail
    • Spiny Astraea snail
  • What fish eat dinoflagellates? – Any fish that has a habit of eating algae or pecking at your live rock could be at risk of consuming dinos and their toxins.


Diatoms are another algae with a brown appearance and can cover everything in their ugliness.

Are diatoms good or bad?

Diatoms are not necessarily a bad thing but they are pretty unpleasant to look at in a tank. For most aquarists who spend hours upon hours trying to get the perfect look for their tank, the presence of the diatoms can really cause some frustration.

Can diatoms kill fish?

Diatoms themselves can’t kill fish but the overall effect that they may have on your water quality could end up hurting your fish. However, that would really only be in the case of a large bloom.

Can diatoms kill coral?

Corals are another story altogether since they are largely stationary most of the time. The diatoms can begin to grow around the coral and up their bodies and eventually kill them if given the chance. If you care about your corals and are beginning to see more and more diatom algae spreading throughout your tank, it is advisable to begin taking care of the problem.

How to identify diatoms

Diatoms are fairly easy to identify. First, is it an ugly brown? next how easily are they removed from a surface? If you have to scrub hard to remove the brown substance from glass or other surfaces, it is likely diatoms. However, it could still be mistaken for dinoflagellates which is why you need to remove some of the substance and place it in a cup, shake it up, and strain the contents out using a paper towel. If the substances begin to come together again, it is likely a dino.

Types of diatoms

There are two groups of diatoms and they are really only distinguishable on the microscopic level.

  • Centric Diatoms – Centric Diatoms come in a variety of shapes and sizes.
  • Pennate Diatoms – Pennate Diatoms are symmetrical and move along solid surfaces. they often have the ability to secrete mucus that allows them to glide across surfaces.

What are diatoms? Are diatoms algae?

Diatoms are algae with a brown appearance that more often than not appears after a tank cycle, though there is the potential for diatoms to appear later in the tank’s life as well.

What causes diatoms? What do diatoms eat?

Diatoms thrive off of the silicates that are present in some water. A good way to prevent diatoms is to use water that has little to no silicates present. You can achieve this by using water that has been treated with a RODI system.

However, silicates are not limited to the water source to enter your tank. Silicates can enter through salt mixes and substrates. The source of the silicates is even more important to identify than the fact you have diatoms in the first place.

Will diatoms go away on their own?

As long as nutrients are present for the diatoms then they will persist. in most cases, you will need to do something on your part to remove them from your reef tank.

How to get rid of diatoms

Luckily, you have some options when it comes to removing diatoms from your reef aquarium. Water changes should be near the top of your list, just make sure that you are indeed dealing with diatoms or you risk causing a bigger problem.

If you have a protein skimmer in conjunction with water changes to make sure it is running frequently to clean up the water as much as possible. You can also use additives to help cut down on the diatom population as well.

  • Will Vibrant kill diatoms – Vibrant is excellent at clearing up the water in your aquarium of any algae including diatoms. Essentially, vibrant introduces new bacteria that go after the diatoms and help to reduce their numbers so, in conjunction with water changes and skimming, your tank will be looking great in no time. Just make sure you are following the manufacture’s instructions for accurate use.
  • Will Phosguard remove diatoms? – Phosguard will attack the phosphates and the silicates in your aquarium which are the diatoms main source of nutrients. Without phosphate and silicates in your aquarium, the diatoms will slowly begin to starve and the population will decrease with protein skimming and changing your water.
  • Hydrogen peroxide dosing for diatoms? – Hydrogen peroxide can kill diatoms but it can also hurt other organisms in your tank so you have to be really careful. We’ll be releasing an article on hydrogen peroxide dosing soon so be on the lookout for it if you want to learn about how to dose it to deal with diatoms and other problems.
  • Will a UV sterilizer kill diatoms? – You can definitely use a UV sterilizer to help clean up the diatom population in your aquarium. Just make sure to keep it running as much as you can and to thoroughly scrape the sides of your tank and clean your filter regularly. You may be decreasing the current population but you ultimately want to make sure that their food source is under control.
  • What eats diatoms? – Since diatoms are a kind of algae, most species that you might expect to eat algae will also eat diatoms. However, it is important to remember that the source of their food which is the silicates and phosphates in your aquarium is what really needs to be taken control of to keep the diatom population low.
    • Do copepods eat diatoms? – Copepods will eat your diatoms if given the chance and with a large enough population of copepods, they can totally swing the diatom population around in your favor. The best part is that they regulate their own population with little to no side effects. If there are too many copepods and not enough nutrients, they just die off.
    • What cleanup crew eats diatoms? – Snails and more snails. many types of snails will assist in controlling the diatom algae removal in your aquarium. Here are a few examples
      • Nassarius snails
      • Astrea snails
      • Banded Trochus snails
    • What fish eat diatoms? – Most fish that eat algae on occasion have the chance to nibble on some diatom algae. Here are a few examples of some common algae snackers.
      • Kole Tang
      • Lawnmower blenny
      • Potter’s Angelfish
      • Red-lipped blenny

Final Thoughts

Knowing whether you’re dealing with dinoflagellates or diatoms is obviously critical to how you will deal with either problem. This exhaustive article hopefully has armed you with every detail you need to handle either outbreak and easily identify each.

Unfortunately, these aren’t the only problems you’ll deal with in a reef tank. If you’d like to continue learning about how to deal with common saltwater aquarium algae I recommend checking out our Reef Tank Algae article.

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