No worries, mate! You needn’t be concerned in the least that hydroids have appeared in your seahorse tank. They won’t do any harm whatsoever to large seahorses like Mustangs and Sunbursts (Hippocampus erectus), even a gravid male. You only need to be concerned about keeping hydroids out of your aquarium if you are keeping dwarf seahorses (H. zosterae) or raising seahorse fry. And in your case, sir, even if your male gives birth in the main tank where there are abundant hydroids, the newborns will face little risk from these pests because they will head immediately to the surface of the aquarium to fill their bladders and congregate at the top of the tank, well out of reach of the hydroids that have sprouted on your live rock and substrate. A diligent aquarist like you will surely notice the arrival of the newborns in plenty of time to transfer them to your nursery tank before they can come to any harm from the hydroids.
So you don’t need to do anything to eradicate the hydroids in your seahorse tank. Of course, you will want to keep them out of your nursery tanks but that can easily be accomplished by treating them with fenbendazole (brand name Panacur), as I’ll explain later in this post.
Sooner or later hydroids will appear in any marine aquarium that is receiving regular feedings of rotifers, copepods, or baby brine shrimp. It’s inevitable because they can can gain entry into the aquarium in many ways. For example, they are notorious hitchhikers. Both the colonial polyp stage and the free-swimming micro-jellies can thumb a ride on live rock, macroalgae, hitching posts, sand or gravel, specimens of all kinds, or within so much as a single drop of natural seawater (Abbott, 2003). Beware of fuzzy looking seashells! Very often hydrozoans come in on the shells of the hermit crabs or snails we purchase as aquarium janitors (Abbott, 2003). Or they may be introduced with live foods, or even among Artemia cysts, in some cases it seems. They can even be transferred from tank to tank in the aerosol mist arising from an airstone or the bubble stream of a protein skimmer.
Once they find their way into a dwarf seahorse setup or nursery tank, hydroids can explode to plague proportions very quickly because conditions are ideal for their growth: perfect temperatures, an abundance of planktonic prey that is renewed every few hours, and a complete absence of predators.
When hydroids become a problem in the nursery or dwarf tank, there are a couple of good ways to eradicate them and get the situation under control again:
Hydroids can be controlled in the aquarium by using a medication known as fenbendazole to treat the tank over a period of days. Fenbendazole (brand name Panacur) is an inexpensive anthelmintic agent (dewormer) used for large animals such as horses, and the de-worming granules can be obtained without a prescription from stores that carry agricultural products (e.g., farm and ranch equipment, farming supplies and products, veterinary supplies, livestock and horse supplies, livestock and horse feed). If you live in a rural area, those would be good places to obtain it as well.
You can also fenbendazole granules in small quantities online from the following vendor:
However, there are a couple of things you should keep in mind when treating an aquarium with fenbendazole, Sean. Administering a regimen of fenbendazole (FBZ) or Panacur will eradicate any hydroids, Aiptasia rock anemones, or bristleworms from live rock or live sand, thereby rendering them completely seahorse safe. The recommended dose is 1/8 teaspoon of the horse dewormer granules (22.2% fenbendazole) per 10 gallons of water. Dose aquarium with 1/8 teaspoon/10 gallons every other day until you have administered a total of 3 such treatments (Liisa Coit, pers. com.). Even one dose will do a fine job of eradicating bristeworms, but Aiptasia rock anemones and hydroids are a bit tougher and may require 2-3 doses to eliminate entirely.
Because fenbendazole is essentially a de-worming agent, it will destroy any bristleworms, flat worms, spaghetti worms or the like. The FBZ or Panacur treatments are best administered to the live rock in a bucket or hospital tank before the LR is introduced in the main tank. Otherwise, the massive die-off of the worm population in the aquarium may require large water changes in order to prevent a dangerous ammonia spike! And after the treatment is completed, its a good idea to add a portion of newly purchased live sand to the system in order to help restore its normal diversity of fauna and microfauna again (Liisa Coit, pers. com.).
Fenbendazole does not have any adverse effects on biological filtration, but be aware that it is death to many Cnidarians besides hydroids. Mushrooms and related corals are generally not affected, but expect it to have dire effects on other corals (e.g., sinularias), polyps, gorgonians, and anemones. In general, any Cnidarians with polyps that resemble the stalked family of Hydrozoans are likely to be hit hard by fenbendazole, so don’t use this treatment in a reef tank!
Also be aware that fenbendazole seems to soak into the porous live rock and be absorbed indefinitely. I know one hobbyist who transferred a small piece of live rock that had been treated with fenbendazole (Panacur) months earlier into a reef tank, where it killed the resident starfish and Astrea snails. So enough of the medication may be retained within treated live rock to impact sensitive animals months after the fenbendazole was administered. Don’t treat live rock intended for reef systems with fenbendazole (Panacur)!
At the lower dosage recommended for nursery tanks and dwarf seahorse tanks with fry (1/16 tsp. per 10 gallons), fenbendazole normally does not harm cleaner shrimp and decorative shrimp. With the exception of Astrids (Astrea), Coit and Worden have found it does not usually affect the types of snails typically used as cleanup crews (e.g., Nassarius, Ceriths, and Nerites). It will kill starfish but copepods, hermit crabs, and shrimp are normally not affected.
Macroalgae such as the feathery or long-bladed varieties of Caulerpa or Hawaiian Ogo (Gracilaria) are not harmed by exposure to fenbendazole at even triple the normal dose. In fact, if you will be using Caulerpa in your nursery tanks to provide hitching posts for the fry and serve as a form of natural filtration, it’s a very wise precaution indeed to treat them with a regimen of fenbendazole beforehand.
So fenbendazole (FBZ) or Panacur is primarily useful for ridding bare-bottomed nursery tanks and dwarf seahorses setups of hyrdroids and Aiptasia anemones, ridding Caulerpa and other macroalge of hydroids or Aiptasia before its goes into the aquarium, and cleansing live rock of bristleworms, hydroids, and Aiptasia rock anemones before it is introduced to the aquarium.
It can also be used to eradicate bristleworms, hydroids, an Aiptasia from an established aquarium if it does not house sensitive animals such as live corals and gorgonians, starfish, Astrea snails, or tubeworms and other desirable worms that may be harmed by FBZ, providing you monitor the ammonia levels closely and are prepared to deal with the ammonia spike that may result from the sudden death of the worm population.
In summation, sir, if the aquarium with a hydroid problem will be housing live corals at some point, it would be best not to treat it with fenbendazole. And you may not need to be concerned about the hydroids in the first place, since as long as you keep them out of your nursery tanks, hydroids won’t present a problem for large seahorses like Mustangs and Sunbursts (Hippocampus erectus) at all. They are impervious to their stings. In fact, in the wild, seahorses often encourage algae, small bryozoans and other encrusting organisms, including hydroids, to grow on their exoskeleton in order to enhance their camouflage (Vincent 1990). The hydroids are only a problem for baby seahorses in nursery tanks or for the tiny dwarf seahorses, which are susceptible to their stings. So as long as you will be keeping large seahorses in the aquarium that has hydroids, there is really no need to eliminate them.
If you still feel it is necessary to eradicate hydroids from an aquarium that will be used for live corals and delicate invertebrates, there is another alternative to killing them with fenbendazole that you can consider. Hydroids of most kinds can be eradicated from the aquarium by raising the water temperature to 92°F or above for period of 3-5 days (Liisa Coit, pers. com.). Keep all of the filters and equipment operating and sterilize your brine shrimp nets, so that the hot water circulates throughout them and destroys any hydroids or hydromedusae that may be present in the filtration system. Be sure to sterilize your brine shrimp nets, etc., at the same time as you are heating up the infested aquarium. (Seahorses and their tankmates, including snails and the cleanup crew, must be removed to a temporary holding tank while the heat treatment is carried out.) Maintaining the water temperature at 92° for this period does not harm the beneficial nitrifying bacteria in your biofilter, injure marine plants or macroalgae, or kill off copepods and other beneficial microfauna (Liisa Coit, pers.com.).
After the treatment period, perform a large water change to assure that the die off of hydroids does not degrade your water quality, and adjust the water temperature back to normal, and all the animals can be returned to the aquarium. The tank will not undergo a "mini cycle" and there will be no ammonia or nitrite spikes (Liisa Coit, pers. com.).
However, not all types of hydroids respond to the heat treatment method of eradication. The snowflake type of hydroids that are all too common seem to have no difficulty surviving the heat treatment. So generally speaking, then Panacur (i.e., fenbendazole) is a more reliable way to eliminate them. But if your hydroids do not resemble snowflakes, then there is an excellent chance that the heat treatment will be effective.
Okay, Sean, that’s the quick rundown on hydroids and when they need to be controlled. They are really not a big deal unless you’re keeping dwarf seahorses.