Hydra in a Shrimp Tank. Treatment.

Hydra vs shrimp Wat fun

Today we are going to talk about another unwelcome parasite in our shrimp aquariums – Freshwater Hydras. Shrimp breeders (as well as fish keepers) consider them to be pests which can cause problems and they are absolutely right.

Hydras are named after the nine-headed sea snake of Greek mythology which re-grew heads after decapitation. Luckily, we have ways to deal with it without chopping off its heads. No Planaria, Hydrogen peroxide, Fenbendazole and etc. can easily remove it from our aquariums. Nonetheless, we need to know more about this parasite to protect our shrimp.  

About Hydra

Hydra is a small-sized polyp from the same phylum (Cnidaria) as sea anemones and jellyfish. While most cnidarians are marine, Hydra is unusual in that it lives exclusively in freshwater.

It was first described by Antoni van Leeuwenhoek (1632–1723) in a letter he sent to the Royal Society on Christmas day, 1702. These creatures have long fascinated biologists due to their ability to regenerate from small pieces.

Remarkably, even cells from a mechanically separated Hydra can restore, and over the course of about a week reassemble into a functioning animal. How this process occurs scientists still do not understand completely.

Types of Aquarium Hydra

Hydra Taxonomy

Several species of hydra have been recorded, but most are difficult to identify without detailed microscopy. Two species, however, are distinctive. They are the most common in our aquariums.

• Hydra (Chlorohydra) viridissima (Green hydras)

  • Hydra (Chlorohydra) viridissima (Green hydras) is a bright green species, owing to the presence of numerous algae called zoochlorellae, which live as symbionts within the endodermal cells.

They are more commonly whitish in color, actually. The green algae carry out photosynthesis and produce sugars that are used by the hydra. In return, the carnivorous diet of the hydra provides a source of nitrogen for the algae.

Green hydra are small, seldom more than 0.4 inches (10 mm) long, with tentacles about half the length of the column.• Hydra oligactis (Brown hydras)

  • Hydra oligactis (Brown hydras) is easily distinguished from another hydra by its very long tentacles, which may extend to 2 inches (5 cm) or more when relaxed.

The column is a pale translucent brown, 0.6 to 1 inch (15 to 25 mm) in length, with the base distinctly narrowed to form a “stem” or “foot.”



Body Structure of Hydra

Hydra structure of the bodyAll freshwater hydras have a radially symmetrical, two-cell layer, tubular body separated by a thin noncellular layer called the mesoglea.

Their combined mouth–anus (gastrovascular cavity) structure is ringed with outwardly projecting tentacles containing stinging cells (nematocysts). This means they only have one opening in their body and that is a mouth but that also serves to get rid of waste products.

Interesting fact: Hydra has the tissue, but it lacks organs. It consists of a tube about 5 mm long formed by two epithelial layers (endoderm and ectoderm). It has a head consisting of a mouth surrounded by a ring of tentacles at one end and an adherent disk, the foot, at the other end. Interspersed between the cells of the epithelial layers are multipotent stem cells that give rise to four differentiated cell types: gametes, nerves, secretory cells, and nematocytes — the stinging cells that define the phylum Cnidaria.

Also, due to the structure, they have the ability to regulate their water inside their bodies. So they can elongate or contract their bodies at any time.
Although it lacks sensory organs, Hydra is responsive to light.

Reproduction of Hydra in Aquariums

Hydra reproduction asexual and sexualHydra undergoes two mutually exclusive modes of reproduction: at warm temperatures (18–22°C or 66-72F) they reproduce asexually by budding. (Spermatogenesis in Hydra oligactis: II. How temperature controls the reciprocity of sexual and asexual reproduction. Developmental Biology Volume 146, Issue 2, August 1991, Pages 292-300).

Reproduction in hydras typically takes place asexually by a process known as “budding”.  A bud-like growth on the body of the “parent” hydra eventually grows into a new individual that becomes separated from the parent.

When conditions are harsh, or there is a shortage of food, hydras can reproduce sexually. A single individual may produce both male and female sex cells, which are released into the water where fertilization occurs.

The egg develops into a larva, which is covered in tiny hair-like structures known as cilia. The larva may either settle immediately and develop into a hydra or become surrounded in a tough outer layer that allows it to survive harsh conditions.

How Fast Hydra Reproduces

The answer is very fast! Under favorable conditions, (it is very unpretentious) the hydra is able to “generate” up to 15 small hydras a month. It means that every 2-3 days it makes its copy.

One hydra, in just 3 months, is capable of producing 4,000 new hydras (considering that “children” also bring 15 hydras a month).

Why Hydra is Dangerous to our Shrimp

In nature, hydra feeds on small aquatic invertebrates that are paralyzed by the stinging cells when the prey comes into contact with the tentacles. The prey is then brought to the mouth by the tentacles and taken into the body of the hydra.

Some of their favorite foods include organisms up to twice their size like Daphnia, cyclops, and other freshwater copepods.

For example, Daphnia, (small planktonic crustaceans) are 0.2–5 millimeters (0.01–0.20 inches) in length. I have already covered the breeding and life cycle of shrimp in another article and it takes 14 days for the baby shrimp to reach 5,4mm in length. Meaning that hydra can easily catch 14 days old baby shrimp! I would say that any shrimplet less than 1-month-old will be in mortal danger if there are hydras in the tank.

Hydra paralyzes the prey with neurotoxins which it releases from tiny stinging organelles, called cnidae or nematocysts. The cnidae is a part of the ectodermal cells of the column, especially the tentacles, where they are packed in high densities.

Each cnida is a capsule containing a long and hollow thread. When stimulated by chemical or mechanical signals, the permeability of the cnida increases. The largest cnidae (penetrants) contain the neurotoxins that hydra injects into prey via the hollow thread.

Smaller cnidae, which are adhesive, coil spontaneously on contact with prey. It takes less than 0,3s to sting the prey.

As a result, when the shrimp comes too close to Hydra, you will notice how they jerk away very quickly. The same happens with a small fish or fry, they swim away very abruptly. The reason is that hydra will “stab” what they think might be a potential predator or prey.

If shrimp are full-grown it does not tend to bother that much. Nonetheless, if you have got a newly born shrimp it can harm or even kill and eat them depending on the size.

Note: Stings from a few hydras are harmless to most people. Nevertheless, if we are talking about thousands and millions it can damage even human skin. For example, on fishing nets left submerged for a long time, the combined stings have been known to cause rashes on the hands of fishermen!

Short Info:

Name Hydra
Type parasitic polyp
Threat level Medium/High
Treatment difficulty Easy
Treatment efficiency 100%
Treatment cost 3-25$
Treatment length Up to 1 week

How Hydra Eats

How hydra eatsHydras are predatory and voracious. They eat worms, insect larvae, small crustaceans, larval fish, and other invertebrates, such as Daphnia and Cyclops.  

Hydra is not an active hunter. They are classic ambush predators that sit and wait for prey to come close enough to strike. The instant prey is close enough to activate a reaction of stinging cells.

It is an instinctive response. Then the tentacles start to curl around and close on the victim, drawing it to the mouth at the base of the stalk of tentacles.

If it is small enough, hydra will eat it. If it is too large to consume it will be discarded, possibly to be found by a mystified aquarist, with no obvious cause of death to be seen. In case there is not enough prey, they can get some amount of food by absorbing organic molecules directly across the surface of their body.

When there is no food at all, hydra stops reproducing and starts using their own tissues for energy. As a result, it will shrink to a very small size before it finally dies.

How can Hydra get into Shrimp Aquarium

Hitchhiking with new plants (quarantine the plants), wood, or rocks harvested from outdoor water sources. They can sneak into your tank as snails do. It is almost impossible to notice them. Especially, when they retract their tentacles they look like a floating piece of sand or a bit of floating debris.

How to Recognize Hydra in Time

Hydra in the tank

  • By the look

Frankly saying it is very hard to notice them in the beginning. At first, they are small and appear initially as little white spots. Then they become like kind of little white stringy things.

Anyway, that is not enough to discern what they are. Unless you are ready to sit 24/7 with a magnifying glass near your aquarium.

Once they start clinging to the glass or a black surface, it will be easy. Their “strange” form will catch your attention. Unfortunately, in most cases, it means that hydras have already spread in your tank. They multiply very fast!

How hydra moves

  • By the movement

Hydras are generally sessile, but they can detach and move to another location by gliding slowly on their base. They move very oddly. Hydra cannot swim like fish or shrimp.

They can either suddenly move by flexing their tentacles or they can kind of somersault. Their movement mimics that of small worms when they do this.

How to Remove Hydra From the Shrimp Tank

Actually, I would like to start off by saying how not to remove them.

Do not squash it with your nail! Remember that they can regenerate themselves. In some ways, Hydra is, for lack of a better term, immortal. Yes, you have heard me right.

Researchers have discovered a set of transcription factors that control the ability of the Hydra to constantly renew all of its cell types ultimately preventing the effects of aging.

Daniel Martinez followed 100 adult Hydra for four years, discarding the buds as they were produced. The parental animals did not undergo age-related changes. Individual cells die in Hydra, but the organism as a whole does not have a fixed lifespan. (Current biology. Quick guide Hydra. Kristine M.GlauberCatherine E.DanaRobert E.Steele. Volume 20, Issue 22, 23 November 2010, Pages R964-R965).

So damaging them is simply counterproductive.

Manual removal

If it is a small infestation you can try to physically remove it. Really the best way to kill them is to grab them and to just physically remove them from your tank. You can also siphon out any Hydra you see. If there are not too many, simply take some care not to damage them while you do so.

Reduce feeding sources

Hydra is a predator. They only eat living things that they kill with their poison. Overfeeding often increases the populations of tiny prey hydra do feed on. Therefore, it would be better to avoid overfeeding, which is a very common mistake with many shrimp breeders. Use feeding dishes for food control.

You can try to starve hydra out. Like most other pests, Hydra can only gain a foothold if they have access to food. Cutting back food and doing target feedings can keep Hydra controlled. That is the long-term solution (at least a few weeks), and it will work.

Adding fish

Hydra themselves are food, for any fish that are willing to eat them. There are reports that Sparkling gouramis, Blue gouramis and etc. will eat Hydra. The problem is that fish is the problem itself for the shrimp tank. Even small fish can wipe the shrimp colony. In my opinion, it is not worth the risk in our case.

Adding snails

There are some snails species that have been reported to eat Hydra, for example:

However, do not think that these snails will specifically hunt down the Hydras. No, it is just they do not care what to eat. Unfortunately, these snails (especially Marisa cornuarietis) can also eat some plants. So, be careful, and do your research beforehand.

The other important thing that most freshwater snails benefit the aquarium is by consuming waste products. Because they are consuming waste products less waste is available for small microscopic organisms that may be a food source for the Hydra.

High and low Temperature

You can try to use a heater to kill the hydra. In this case, you will have to remove all your shrimp and fish from the aquarium if you do not want to kill them all. After that, raise the temperature to 105 – 110 degrees Fahrenheit (41-43 C) for several hours.

Most plants should survive this treatment. It is not a long time and should not be enough heat to harm most of them. However, if you have any very sensitive or very highly valued plants, I would take them out and place them into separate containers of water. 

Another way is to put it in very cold water. Unfortunately, it can go dormant oftentimes rather than die.

Light Trap

Hydra is responsive to light that is why you can see a lot of them on the front glass of the aquariums. Use it against them. You need to:

1. Shade your aquarium or turn off the light completely.
2. Add another piece of glass to the glass from the inside of the aquarium.
3. Use the flashlight or any other source of light so the light will go through these 2 layers of glass.
4. Wait a few hours.
5. By the end of the day, the hydras will move to the light and sit on this piece of glass.
6. Carefully remove the second glass with the hydra on it.

This method will not remove all hydras. Nonetheless, it can reduce the number significantly. It is better to combine it with other methods.

Medication against Hydra

Hydrogen peroxide (H2O2)

Hydrogen peroxideHydrogen peroxide (link to check the price on Amazon) (H2O2) is an oxidizing agent and is often used to kill bacteria on a cut or wound.

When applied, the enzymes present in cells act as a catalyst and H2O2 starts breaking into water and oxygen atoms, which badly need second electrons. With the electrons being stolen, the cell wall gets damaged and the bacterium dies. This is also how it kills Hydra.

Warning: DO NOT overdose. Check and double-check your calculations. Overdosing can kill your shrimp.

I have found that the safe level is 14.3 μl H2O2/L, approximately 50 % of the largest applied dose that caused no mortality (NOAEC), which was 29 lL H2O2/L. (“Short communication: Acute toxicity of hydrogen peroxide in juvenile white shrimp Litopenaeus vannamei reared in biofloc technology systems” Plinio S. Furtado, Fabiane P. Serra, Luis H. Poersch, Wilson Wasielesky Jr. Aquacult Int (2014) Received: 6 March 2013)

Measure the water volume of your aquarium. Take into consideration (approximately) substrate, decorations, etc.

  1. Turn off your filter
  2. Use 3% Hydrogen peroxide at 1.5ml per gallon or per 4.5 liters.
  3. Evenly spread H2O2 across the surface of the aquarium.
  4. Gently stir the water to spread it.
  5. Wait for 60 minutes
  6. Turn your filters back on.
  7. The same week do a water change and add your favorite bacterial supplement back to the tank.

Note: besides killing hydra it will give your tank a fresh start but with a filter that is already cycled! Unfortunately, you can lose some biofilm but it will not disrupt the cycle anyway. Even more, if you use a bacterial supplement it will grow back very quickly.

Big thanks to the Marks Shrimp Tanks for this method. According to him, it can also cure “Ellobiopsidae or Green fungus“!.

 Sea salt or Iodine-free table salt

Aquarium salt“Also, sea salt or iodine-free table salt is supposed to destroy the Hydra. Therefore, you can give 20g on 100 liters of aquarium water. It is not dangerous for plants and shrimps according to the statement of shrimp holders. Even in higher proportioning above a few hours”. I have taken it from the German professional site about shrimp.

Freshwater aquarium salt – link to check the price on Amazon.

No-planaria, Planaria Zero, Canine Dewormer safe-guard, and Panacur


Medica alll No-planaria, Planaria Zero, Canine Dewormer safe-guard, Panacur ® Suspension 10% for dogs are also effective against hydra. The dosages are exactly the same as with Planaria parasite.

You can see the dosages and read more aboutPlanaria and Shrimps. How to remove themright here.  There is detailed information about it.


There is no reason to panic if you have found a hydra in the aquarium. They will not annihilate your shrimp colony in the blink of an eye. However, it would be better to make preparations to deal with the Hydra as soon as it is possible. So as to reduce any potential losses. 

Read also my article “Understanding Dwarf Shrimp Diseases and Parasites”.


17 thoughts on “Hydra in a Shrimp Tank. Treatment.

  1. Treated my 5 gallon metaframe with a cup (approx 236 ml) of h2o2. I removed the shrimp prior. I havent seen any hydra in a week. Got rid of the nuisance snails as well. The plants did survive. A bit overkill but hey, pretty safe to say it worked. Thank you for the info!!

    1. Hi Timothy,

      You are welcome!
      Glad that it helped and thank you for the feedback.

      Best regards,

  2. i going to use sea salt i use fine ground sea salt in my cooking its pure so fingers crossed thanks for the advice

  3. Can the peroxide method be used in a tank with snails? I have mysteries, rams and netites. Thanks for your help!

    1. Hi Bethney Bogan,
      Hi, yes, you can use peroxide with snails, just make sure that it does not get directly on the snail’s skin!
      Best regards,

  4. Hello Michael,

    thanks for the advices, solved my hydra overpopulation without further damaging the ecosystem (didn’t want to use copper and other “hard” toxins)!

    Peroxide method seems to be the most safe, just don’t do it in low-tech aquarium without external biofilter unless you have bacterial supplements ready. Removing significant portion of the biofilm might result in rapid changes in water chemistry, which can cause algae bloom or in worst case even nitrate toxicity.

    Also I would like to point out an error in the text – according to referenced study, safe level of H2O2 per liter of water is 14.3 μl (microliter), NOT ml (milliliter)! Better to update it to prevent possible mistakes, since adding 14 milliliters per liter would be fatal for everything in the aquarium I guess…

    Have a nice day!

    1. Hi Jiri,
      I am glad that I could help you.
      Thanks for noticing the spelling mistake!
      Best regards,

  5. Hi Michael I am using the 3% hydrogen peroxide method. I have a question. Do I do sit only once or do I do everyday or every other day?. Why does this thing keep saying my comment is a spam?

    1. Hi Alecia D Earnest,
      Sorry, that your message was marked as spam. There is a very strict filter on my site. Otherwise, I would be overwhelmed by spam.
      Unfortunately, sometimes normal messages are also getting into this category.
      Nonetheless, I still try to check everything.

      Regarding your question, – you need to do it once. It should be enough to kill Hydra.
      Best regards,

  6. I have a 4 gal tank. Its full of plants it’s probably 3 gal of water plus a large driftwood. A big hide and lots of moss. So maybe a 2 half gal of water volume.. lots of baby’s 50 to 60 shrimp. 3 mystery snails. How much peroxide would I use?

    1. Hi Angi Beverly,
      For 2.5-gallon tank volume, you can use 3.75 mL at most.
      Nonetheless, because it can be hard to measure hundredths, I would play safe and lowered it to 3mL.
      Important, these are calculations for 3% Hydrogen peroxide.
      Best regards,

  7. Hello, I really appreciate your site for info on all things shrimp and snail. Is hydrogen peroxide the only safe treatment for a tank with both shrimp and snails? I’ve just noticed some tiny hydra in a few of my tanks and want to knock it on the head before it’s out of control. My tanks are approximately 38 Litres each, how much should I dose with?

    1. Hi Helen,
      Unfortunately, I don’t know any other method that can be used against Hydra in shrimp and snail tanks at the same time. Maybe anybody else can suggest something else?
      As for the calculations, you will need 12,6 ml for 36 liters. However, I would lower this number to at least 12 (1.5*38/4.5=12.6) (because of the filter, substrate, air pump, decor, or whatever else you may have in the tank)!
      Best regards,

      1. Thanks. I tried dosing my tanks as recommended and I’m afraid the hydra were still hanging on afterwards. What might have gone wrong?
        Also with the filter off for 1 hour instruction I’m curious about how long the h2o2 remains potent in the water. Is it spent after 60minutes?

        1. Hi Helen,
          There are many factors that influence the decomposition of Hydrogen peroxide. For example, low pH, high temperature, and light will increase the process.
          According to the experiments, H2O2 loses most of its potency in 20-30 minutes, after that the process slows down. Therefore, generally, 60 minutes should be enough. In addition, we don’t want to keep the tank without filtration any longer.
          Best regards,

  8. Hi Michael!

    Great post!
    Thank you for your contribution with this information.

    I have a question:
    I have a 100l tank with live plants, snails and an axolotl swimming in it, and recently I found out that I have a few hydras on the plants, but as of today there are hundreds if not thousands of them all around the tank.
    I want to get rid of them, because I am planning on adding more plants in the aquarium so that I could add shrimp to this tank in a few months. I understand the obvious- the axolotl will eat the shrimp, but this is why I will heavily plant the tank before I do so, so the shrimp would have enough places to hide from the giant creature.
    The problem is that the hydras will eat all of the shrimp babies that the shrimp will eventually carry and I want to fix this before it even happens.

    How did the hydras start reproducing so fast in my tank? Before I added new plants (which I suspect were carrying these critters with them) I had a HUGE population of cyclops living in this tank, like they were everywhere, and I didnt mind them, because they were sort of helping to clean the aquarium, and then the hydras came and now what is left of that giant cyclops bloom is just a few odd cyclops swimming here and there, but their population has definitely been devasted in a course of a week.

    So my problem is this- I cant add any new fish that would eat the hydras, because the fish will be quickly exterminated by the axolotl. I can’t add any pond snails, because I do not want to transmit any parasites to my axolotl when he finds out that he can eat the pond snails as well (he’s already cleared out the entire population of ampularias and now he’s quickly getting the spiraly (horn snails I believe) (the common snails that inhanit aquariums) snail population down as well). And manually removing the hydras wont do as well, because, well they are now everywhere and it is just not possible to pick each individual hydra up, because there are so many, they are even frequently floating around in the water.

    So I presume that the only option for me is to do chemical warfare.
    Now I would very much prefer that the chemicals wouldn’t decimate my snail population, because they keep the algae’s at bay and at the same time feed my axolotl (I know, I have a nice balance in the aquarium where everyone feeds everybody else :D), however taking the snails out while there are chemicals in the water is also not a great idea, because I’ve seen that the snails are also carrying the hydras on their shells, therefore I do not want to run into another infestation once the snails are re-introduced back in the tank.

    At the time being the safest bet for me seems to be the hydrogen peroxide method, however I am worried about the axolotl (it is known that they have very sensitive skin) as well as the plants (I have no clue if plants are hardy enough, I have some anubias, floating plants and anither type, which I sadly dont rember the name of) and snails.

    The other method is to use Planaria zero, but sadly this item is not shipped to Latvia and in Latvia we don’t have any deworming agents that are in powder form 🙁

    What are your thoughts and suggestions for me?
    If you think that the hydrogen peroxide method is the best for me, then what dosage do you suggest (I am also a bit conflicted- do I use microliters or mililiters with this method).

    Many thanks in advance!

    1. Hi Janis Bukoveckis,
      Thank you!
      I need to say it right from the beginning – do not add shrimp to the axolotl tank. This is not a good idea. They will be eaten eventually. Do not think that a heavily planted tank can change it. Sorry.
      As for the hydrogen peroxide. I would not worry about Anubias, this is a hardy plant. Floating plants can be a bit sensitive but it also depends on the species.
      Personally, I would not risk using hydrogen peroxide with the axolotl.
      In my opinion, it will be better to move your axolotl to another tank before treating hydras.
      Best regards,

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