Hydroids in a Seahorse Aquarium

What is a Hydroid? 

Hydrozoa are small predatory animals commonly found in saltwater environments. Freshwater species of Hydroids also exists. There are many species of Hydrozoa including solitary and colonial forms. Hydroids are similar and closely related to jellyfish. Some have both polyp and free-swimming medusa life stages. Hydrozoa feed with feeding tentacles that contain stinging cells. These stinging cells can harm seahorses. In small numbers you do not typically have to worry about Hydroids with the larger seahorse species. Seahorse fry (under about 2-2.5 inches) and Dwarf seahorses can be harmed by these small stinging predatory animals.


Hippocampus kuda, common name Spotted seahorse

Do Hydroids Come from Brine Shrimp Eggs??? A Common Misconception and How Hydroids Become a Problem in Fry Systems and Dwarf Seahorse Aquariums

Hydroids being introduced to an aquarium or fry system from brine shrimp eggs is a common misconception. There is no life stage of Hydroids that can withstand desiccation (surviving on dried brine shrimp eggs).

Hydroids are common in many aquariums and often come in as hitchhikers on live-rock, "live" sand, corals, inverts ect. Since many are microscopic they often are not noticed until you have a population explosion. In a typical saltwater aquarium with live-rock, live-sand, corals, ect hydroids are often found in small numbers and the population stays under control as there is limited food for them. In this case, there is not much to worry about as they are very unlikely to sting your seahorses or cause any harm.

In your average display aquarium you often do not see hydroids and they do not typically cause problems. However, in fry systems or Dwarf seahorse systems which are heavily fed Artemia nauplii, hydroid populations can explode and become a problem. There are so many different types of Hydroids with different life cycles that they can be seen quickly or take a while to be noticed when present in a fry or Dwarf seahorse system. Contamination from other systems one may have in their home can also introduce Hydroids. It really only takes a drop of water or less to introduce these tiny microscopic animals to a system. Artemia nauplii and the "messiness" that comes with feeding Artemia nauplii are a perfect food source for a lot of Hydroids which is why they are commonly a problem people run into when feeding Artemia nauplii but the eggs are not the source of these hydroids.

Hydroids can sting and irritate fry and Dwarf seahorses so you do want to avoid them and treat for them if they become present in your seahorse fry or Dwarf seahorse tank. To avoid them, starting with dry sand, artificial decorations, and no live rock (cycled ceramic media), and avoid contamination from other aquariums will help prevent them. Panacur (Fenbendazole) can help treat them but invertebrates can be sensitive to this medication. This will be discussed more later.

How do you Know if you Have a Hydroid Problem in Your Aquarium?

Hydroids are problematic when present in seahorse fry and Dwarf seahorse aquariums. In a typical large seahorse species display tank Hydroids are very likely to present and generally are not harmful. While Hydroids sting and irritate Dwarf seahorses and seahorse fry they do not have a strong enough sting to usually irritate larger seahorses. Since large captive bred species are eating frozen Mysis shrimp, rather than Artemia nauplii, there is a low food source for Hydroids so the population stays in check.  

Since there are so many different types of Hydroids, identifying you have a hydroid problem in your fry or Dwarf seahorse tank can sometimes go unnoticed until the population has exploded unfortunately. Hydriods in a system often appear as white grainy or filamentous. Sometimes they are just seen on the decorations and glass. Hydroids can also grow on your seahorse fry and severally irritate, sting, or even smother them. Treating as soon as you see the hydroids is very important and taking every precaution possible is key to preventing this issue.

Using Fenbendazole to Treat for Hydroids in Seahorse Fry and Dwarf Seahorse Systems

Fenbendazole, brand name Panacur, generally works well for eliminating Hydroids in a fry system. Another brand name we use is Merck Safegaurd goat dewormer (Fenbendazole). We prefer the liquid 10% suspension form rather than the granules.

Fenbendazole is actually a dewormer and is commonly available at tractor supply stores for larger mammals. It is available online as well and is a good medication to have on hand if you are raising seahorse fry or keeping Dwarf seahorses. Seahorses in our experience handle this treatment very well when done properly. It can also be used in a Dwarf seahorse system but invertebrates (corals and cleanup crew) are sensitive to this medication and should be removed from the system during treatment. Fenbendazole does leach in the substrate so reintroducing the inverts may be problematic after a treatment. Macro algae can also be sensitive to this medication.


Fenbendazole does not have any negative effects on beneficial bacteria so you do not need to worry about it causing harm to your biological filtration. It is a milky liquid so some cloudiness of the water during treatment is expected. The protein skimmer and UV sterilization need be turned-off during treatment. Chemical filtration such as carbon must also be removed during treatment. 


The recommended dose for controlling Hydroids in fry and Dwarf seahorse systems is 0.1-0.2 mg per gallon (0.1-0.2 mL per 10 gallons). Repeat treatment every other day for a total of 3 treatments. In some cases longer treatments may be necessary such as 5-6 treatments.

Since part of the life stage of Hydroids is attached to surfaces, wiping down the surfaces in the aquarium frequently will help during treatment. In some cases, when possible, moving your Dwarf seahorses or seahorse fry to a fresh clean system and treating the new system with Fenbendazole can be more effective. When you transfer the seahorses to the new system you can still transfer hydroids so continuing treatment when you are dealing with a hydroid problem is highly recommended. As mentioned before, it really only takes less than a drop of water to transfer Hydroids. A clean system helps reduce food sources for the Hydroids and it is much easier to eliminate them when in small numbers. This will also reduce irritation and stress with your seahorse fry or Dwarf seahorses. 

Caution: Fenbendazole is a dewormer. If for some reason you have a high number of worms in your system such as bristle worms, this medication will kill them which could cause an ammonia spike. Anytime you are running a treatment it is important to watch your water parameters very closely for spikes.

After Treatment one should do a large water change and add fresh activated carbon to pull out the left over medication. At this time you will also want to turn your UV sterilizer and protein skimmer back on if you have these on your aquarium.

Hippocampus zosterae, Dwarf Seahorses

Prophylactically Treating for Hydroids and Preventing them 

If you are concerned about Hydroids becoming a problem in your Dwarf seahorse aquarium or fry system it does not hurt to run a Fenbendazole treatment to be safe. Seahorses handle this treatment very well in our experience and it does not take harm beneficial bacteria so no worries with this. This is assuming you do not have invertebrates, corals or macro algae in your system. 

Formalin for Treating Hydroids?

In a seahorse fry system 37% Formalin dosed at 1 mL per 10 gallons every other day for 3 total treatments may also be effective at eliminating Hydroids. We do not mix this with Panacur. If Panacur is not effective or you do not have this available or on hand using Formalin may be an alternative for treating Hydroids. Seahorses handle proper Formalin treatments very well and this is also effective against common protozoa issues associated with seahorse fry. Fry systems are generally setup "artificial" meaning no other invertebrates (corals or clean up crew) or substrate. Invertebrates are sensitive to Formalin. Some slight cloudiness during treatment is common when using Formalin and is not a concern. If you get high cloudiness a 25-40% water change will help.

Ways to Prevent Hydroids 

Starting with dry sand in your Dwarf seahorse aquarium and dry rock helps prevent hydroids from coming in as hitch-hikers. This does prolong your aquarium cycle time usually. Going with artificial decorations rather than live corals or macro-algae also helps prevent introduction of these unwanted pest. 

For fry systems, washing your hands before working in the system to prevent cross-contamination from other systems is good practice. Prophetically treating as discussed earlier can also help in some cases. Keeping surfaces wiped down reduces risks as well as regular water changes and good filtration. Keeping the bottom on your fry tank well siphoned is important. It is important to remember Hydroids eat Artemia nauplii and waste so keeping the system clean is key. Having your Artemia cultures in a separate area away from other other systems is recommended to prevent accidental contamination.

We hope you find this article helpful. Every system is unique and different. Please take precaution and ask questions before adding any medications to your system if you are unsure. Hydroids are tiny and microscopic and can be confused with other issues such as bio-films. Incorrectly using medications can lead to problems. If you have any questions feel free to reach out to us, we are happy to share our experience and help. Your questions help us improve our content and let us know where we can clarify for others. We appreciate the questions and feedback you may have. 

Email: alyssa@seahorsesavvy.com

Phone: 410-618-3604