Hydra in Freshwater Aquarium


Hydra is not algae or plant, it is a freshwater animal with feeding tentacles such as its cousins living in the saltwater. Hydra has a very simple structure consisting of a tubular body, a “head” at the top, and a “foot” at the bottom. Its tentacles are toxic to many organisms when exposed. These tentacles also can determine if the caught organisms or particles are food or not. The number of tentacles can be four to twelve which extends from the mouth gap. It is relatively small and around only half a centimeter long on average.


The body of Hydra consists mostly of water and it can extend or shrink its body and tentacles. In case of sensing any danger, it pumps out the water inside the body gap and shrinks all the tentacles and body.

It likes the water current which enables more catchable organisms to free-floating in the water column. Hydra uses its tentacles for feeding by extending the tentacles and waiting for the tiny organisms or organic particles to touch them. Then it brings the food to the mouth gap and ingests. The digestion occurs in the body of the hydra. Anything that is not digested will be pushed out from the mouth.

Hydra attaches itself to the surfaces with its disk-like footpad. If it needs to move, it just releases this foot and move with the current then cling again to another surface. For smaller moves to adjust its position for the best feeding place, it uses its tentacles to grab the surface then release the bottom end and flip over itself to stick back to the surface.

It has an incredible regeneration ability. Hydra can regrow a lost head, a foot or both. While reproducing asexually it does not show any signs of senescence (the process of aging). Due to this characteristic, it is very popular in the researches of geriatric medicines.

Hydra in Aquariums


Hydra mostly comes to aquariums by the water of newly added fish from aquarium stores or with plants and rocks added later. Any plants, rocks or livestock transferred from these sources may carry hydra into the aquariums. They may also come with fish foods. One or more of the organic ingredients in the fish foods may contain hydra eggs and if the food is not processed well during the production stage the eggs can survive and enter the aquarium. Then the hydra population increases rapidly in case there is an abundant and continuous food supply.

It can easily maintain in a freshwater aquarium and even doesn’t need any filtration or aeration. Ammonia level increase is hazardous for the hydra such like any other aquarium organism. Chlorinated tap water will usually kill hydra as well as the other smaller creatures upon which they feed and harm the other aquarium inhabitants.

In the aquariums, they can prey on small crustaceans, such as Daphnia, Baby Brine Shrimp, Shrimp fry and Cyclops. Even if there is no prey exists, they can survive by obtaining a certain amount of food by absorbing the organic particles in the aquariums. So, hydra has a large selection of food sources and thus it can survive and reproduce very fast in an aquarium.

Hydra is not harmful but disturbing for larger fish or shrimp but it is deadly for the smaller fish and shrimp fry. The tentacles hurt all inhabitants in the aquarium and the hydra can catch and eat the fry of small fish and shrimp. Due to this reason, it is not wanted in the aquarium hobby for most cases.


Hydra has three reproduction ways which are sexual reproduction, budding, and regeneration.

Sexual Reproduction:

If the environmental conditions get bad, the hydra switches the reproduction method to sexual, which increases the genetic variation in the population and enables long-term eggs to build new offspring when the conditions turn to normal.

When hydra reproduce sexually, simple male organ and ovaries or both form on the bodies of an individual. Sperm released by a hydra into the water column enters the egg within the ovary of another hydra. After fertilization, the egg forms a chitinous exoskeleton and waits for proper conditions before the organism comes out.

Budding Type Reproduction:


Budding occurs when conditions are favorable; proper water temperature and enough food are available constantly and they are the major driving factors for the process. A miniature adult hydra will grow out from the body of the parent hydra as like a plant giving shooting from the body. First, a bud begins to form on the tubular body of the parent Hydra. Then bud develops a mouth gap and tentacles. Once the new bud is fully developed, it detaches from the parent’s body, attaches itself to another surface and continues the life cycle independently. This type of reproduction is extensively used by hydra due to its ease and possibility to create more hydras in a short time.

Regenerative Reproduction:

This method of reproduction is more of a survival mechanism. When a hydra is cut or break into segments, each segment, if it is large enough to provide enough energy and organic matter, will grow into a separate individual hydra, as it happens in the starfish. The size of the new individuals is relative to the size of the segment they formed.

How to Treat / Remove Hydra

If the hydra once gets into an aquarium it is not easy to get rid of but of course possible.

Physical/Semi-Chemical Removal

If the infestation is at the early stages physical removal can be possible. In that case, you may remove all decorations, plants and other equipment and put them into a %10 bleach solution for 15-20 minutes (%5 bleach solution and 10 minutes for the live plants). After soaking in the bleach solution scrub the decorations and plant leaves. Don’t forget to be gentle with the live plant leaves. Then rinse well and let them air dry (except the live plants).

But in that case, some hydra may remain in the substrate or at the tank walls and inside the filters. To be certain about getting rid-off them, the fish and other inhabitants can be moved to another tank first. Then the filters need to be cleaned mechanically. Start with the sponges and rinse them well with water then let them air dry (sun drying works better). Rinse and scrub all the filter parts and buckets, rinse the biological filter media and dry out all these materials and parts. Change the filter wool with a new one since it is hard to clean and easily reachable at affordable prices. Then flush the tank water while scrubbing the walls. Take out the gravel and wash very well. Then let it sundry as well as the aquarium during this period. After cleaning and drying all, they can be set back again as before. After that, move the livestock back to the aquarium.

During the gravel and filter media cleaning, the use of a bleach solution can be effective. But, due to its hard to remove characteristics, we don’t suggest it and don’t find it necessary.

This method is non-practical and harder than setting up a completely new tank. But we must keep our equipment permanent and get rid of hydra as well. So, we have to do these all if we chose this method instead of others.

We don’t prefer this method since it means lots of work and there are other practical ways. This method is mostly used under mandatory conditions if the other methods are not applicable or proper.

Increasing Temperature

Increasing temperature can be also useful in case you can move the fish and plants before application. After removing the livestock increase the temperature up to 35-40 °C. For sure this will have side effects like killing the bacterial ecosystem in the aquarium, but it is one of the methods that can be used. After keeping the temperature for around 8 hours, siphon the bottom lightly to remove dead hydra and other residues. Flush around 50-60% of the water and turn the heater back to the regular temperature and make sure the temperature dropped to normal levels.

After the heating up application, making a filter cleaning is beneficial. Remove all the filter media, sponges and filter wool then rinse all well. If possible, change the filter wool for the best. It is not necessary to dry before re-use. Then it can be set back and ready to run. Before adding the fish and other livestock back, cycle the tank empty for at least one week, add some bacterial additives if possible. If you don’t have the chance to keep fish away that long, try to feed less for the following month. Always keep in mind the filtration is the most important part of an aquarium.

Salt Addition

Adding salt to your freshwater tank will also help killing hydra but this method sometimes is not proper to use due to the livestock sensitivity to salt. But, in case the fish can tolerate salt, it can be a good option.

For preventing shock on fish and other inhabitants, except the hydra, start adding salt gradually. Add 1 teaspoon for 20 liters of water and let them adapt for 1 hour and then repeat the process. Try not to exceed 5-10 grams per liter according to the type of fish. Also, please note that these amounts mentioned there are not always valid. According to the species toleration, it can be increased or decreased. There is no certain point in adding salt to kill the hydra. It should be closely observed both fish and hydra and decided while applying the process.

Due to its uncertainty of the safe salt levels for the livestock, I don’t recommend this option for inexperienced hobbyists. While applying this method keep in mind that it is always better to stay on the safe side and dose less. So, do not add too much salt while trying to kill the hydra. Instead, you can combine this method with the fish addition method to eliminate hydra.

Adding Fish and Snails

If, the methods above don’t fit you or you don’t like them, adding some fish or snails may solve the issue for your case. To eliminate Hydra, adding some fish that will eat them such as three Spot (Blue) Gouramis, Sparkling Gouramis, Paradise Fish, Galaxy Rasbora or Mollies is another option. Pond snails (Lymnaea stagnalis) will also gobble up Hydra while they are don’t receive any other food. The critical part of this application is not to feed the fish too much to force them to eat hydra. So, reduce feeding if you want fish to fight your hydra problem.

Adding fish and keeping them continuously will mostly help with the hydra problem. But, if you have shrimp or other fish fry in the tank, these fish added to the tank surely prey on them too. This could be also considered while deciding to use this method.

Passive Treatment

Hydra mostly reproduces in cases where excessive and tiny food particles existing and permanent in the aquarium. Feeding fish with brine shrimps, water flea, powdered fish food and such food sources will increase and support the hydra population. To prevent its reproduction and being the food source should be cut. If food is unavailable, reproduction will halt, hydra will start using its tissues to live, then start to shrink to a smaller size and it finally dies. To keep the hydra away, after that time you must adjust the feeding amounts, pay attention to good filtration to eliminate fine particles that the hydra feeds on.

Chemical Treatment

The last method which should be considered as the last resort to the hydra problem is chemical treatment due to its risk of affecting the fish, snails, shrimp and plants. Also, chemical treatment may mostly impact the beneficial bacteria. This will cause a halt of the filtration system.

For chemical treatment, there are some deworming agents available like Flubenol and Panacur. These medications kill hydra but at the same time they have a toxic effect on snails and they settle in the soil for a long time. They can be still active after months passed overdosing. This may have one beneficial effect and one side effect on the aquarium. The long-term effectiveness and existence of the medication will surely kill hydra and any possible remaining eggs/offspring as well. This will provide a complete eradication of hydra as a benefit. But, simultaneously it will bring long-term effects on the livestock. While dosing these medications all snails must first be removed.

While using Flubenol is lethal to all snails in the aquarium, Panacur is relatively less lethal to some species like physidae, red-rimmed Melania and great ramshorn and they can survive in correct Panacur doses applied. But, in the case of overdosing on Panacur, some side effects like infertility and formation disorders can occur in shrimp species.

Dosing Panacur:

Active Ingredient is fenbendazole(222mg/gram). The target value to use Panacur is 0,6 ppm and over. To reach that value solve 1 gram of Panacur in 370 ml of water. Every 1 ml of this solution is enough to dose 1 liter of aquarium water. Before dosing with Panacur turn the lights and UV filtration off because it is reducing the effectiveness of it. While dosing, try to spray over hydra. After 3 days from dosing make a %40 water change and siphon the bottom. Follow this cycle at least three times. If the hydra still survives after the second treatment with the same dose increase the dose to %150. This means use 1,5 ml of solution for every 1 liter of aquarium water.

Dosing Flubenol:

Active ingredient is Flubendazole (%5 = 50mg/gram). Mix 1 gram with 100ml of water and shake well to completely solve. Then dose 1 ml of the solution to treat 5 liters of aquarium water. Make water change after three days of dosing by siphoning the bottom at %40 levels of the tank volume. Repeat treatment 2 more times and make water changes as mentioned.


After treating hydra with Flubenol or Panacur, make sure to clean filters and siphoning the bottom. Change the filter wool, use activated carbon in the filter for the following 5 days. Syphon the gravel deeply to remove any remaining medications and change %50 water changes after the last treatment. Repeat deep siphoning at least three more times every following day with %25. This will help to remove as much as possible of the medication remaining in the substrate.

Alternative Chemical Treatment

There are other commercial products available at the market claiming they are safe for the shrimp. You can find below some of them, but the options are not limited to those mentioned below. It is possible to find other products by making a web search. Below given details are derived from the product listings available online.

The BENIBACHI Zero Planaria:

It can help to get rid-off the planaria and the hydra without the upheaval of plants in your aquarium. It does not affect shrimps. (mentioned in the listings)

Ingredients: Betal Nut Palm Extract
Warnings: Provide stronger aeration than usual when in use, do not overdose. Remove snails before treatment.


This product requires 3-day of dosing cycles. If pests persist, repeat a 3-day cycle with a 1-week interval. Make dosing on the first and third day. Wait for another day and do a 20% water change. Dose one last time after 7 days to make sure all planaria and hydras are eliminated. Wait for another day and make a 20% water change. Finish treatment!

Day 1 – Dose 1 spoon per 25L
Day 3 – Dose 1 spoon per 25L
Day 4 – 20% water change
Day 10 – Dose 1 spoon per 25L
Day 11 – 20% water change

Genchem No Planaria:

It kills planaria and hydra without affecting shrimp. It kills some snail species as well.

Ingredients: Betel nut powder
Warnings: Remove snails especially Malaysian trumpet snails and nerite snails, do not overdose, increase aeration.


When using No-Planaria, it is recommended to stay under the dosage on the package. Remove carbon from the filter before dosage and dose according to package instructions. During the treatment period, do not feed the tank. After 72 hours perform a 25% water change. Put active carbon back to filter for a few days following the application.


As it is said for some cases hydra does not suddenly appear due to poor tank conditions. It needs to get introduced into the tank in some way. The high nutritional live foods or such small creatures will encourage it to overpopulate in the tank. With the availability of constant food sources, Hydra can easily reproduce in a short time and reach a population level that may create disturbance and harm to small inhabitants.

Nevertheless, most of the hydra is relatively small in size and not a big threat to most aquarium inhabitants. Usually, a big-scale infestation is disturbing for the fish and shrimp due to the poisonous tentacles that hydra has. But sometimes hydra may feast on little fish and shrimp fry and needs to be taken under control. For smaller infestations no big action is necessary just if you pay more attention to interrupt the food source, the hydra will disappear soon. Thus, avoiding overfeeding is the most critical point preventing hydra to take over the tank and kill it once if it is introduced to the aquarium.

As usual, the best and more practical action is to prevent hydra from getting into the aquarium. In the below, there are some actions listed but not limited to them for preventing the hydra to enter the aquarium.

• Use dedicated fishnets for every aquarium you have,
• Inspect and treat new decoration or plants with saltwater before adding to the tank,
• Try to use frozen live foods and high-quality dry foods since live foods and improperly processed foods may contain hydra or eggs. Growing your live food will be a good alternative,
• Dipping tools like tweezers, scissors, etc. into a saturated salt solution or permanganate solution before using,
• Prefer Immerse produced or In-Vitro aquarium plants.

Image Source: https://fishlab.com/

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