Yes, you are quite right to be concerned about the translucent creatures with the white centers and white appendages. They are indeed a hydrozoans, the mobile medusa stage of a hydroid, which resemble tiny jellyfish. The problem is that they sting just like jellyfish too, and if your seahorses are trying to eat them, mistaking them for copepods or other tasty tidbits, they are a danger to your ponies. The seahorses are not at risk only when they snik up these micro-jellyfish, mermaid — they will also be stung anytime they brushed up against them are come in contact with one of the medusae micro-jellyfish. They are bad and you’ll need to eradicate them from your seahorses take.
Ordinarily, hydroids are only a problem for newborn and juvenile seahorses but any time your seahorses accidentally ingest one of the medusae they are in danger of suffering a serious injury.. And the irritation from a serious hydride infestation would certainly be a source of stress for the ponies, which could make them susceptible to bacterial infections and other problems.
As you know, mermaid, hydroids are always a concern for dwarf seahorse setups as well as nursery and rearing tanks that are receiving regular feedings of Artemia nauplii, rotifers, or copepods. 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 gain entry into the aquarium in many ways. For example, they are extremely accomplished 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.
It can be very challenging to identify hydroids because there are about a zillion different species of hydrozoans and the different types have different characteristics and are often vary remarkably in appearance. There is considerable variation within the species as well, and the same type of hydroids can appear vastly different depending on the size of the colony and its stage of development, conditions in the aquarium, and their predominant diet. And, of course, the different stages of the life cycle of these amazing animals are so entirely different that they were long believed to be different types of cnidarians altogether, and different species names were often assigned to the same hydroid in different phases of its life cycle. Because they are so difficult to identify and are not easy to distinguished with the naked eye during their initial stages, hydroids often go undetected in nursery and rearing tanks until they begin to take a toll on the fry.
The typical hydroid colony has a stem with a variable number of polyps growing on it, and each of these polyps bears numerous tentacles that are liberally studded with knobby nematocysts (batteries of deadly stinging cells). There are many different kinds of hydroids and they appear in the aquarium in many different guises: many colonies are stalked; some have fingerlike projections, others look like tiny pink fuzzy balls or appear like cobwebs (the webbing kind usually spread along the bottom or grow on the aquarium glass along the substrate). The "snowflake" type of hydroids seem to be particularly common in aquaria, whereas other species look more like crystal chandeliers, and some species form bushy colonies as they grow that serve as microhabitats for Caprellid skeleton shrimp and other tiny crustaceans.
Even a large hydroid colony appears harmless to the naked eye of the untrained observer. It takes a much closer look to reveal the dreaded ‘droid’s lethal nature, as described below:
"Studying the colony under high magnification, one soon becomes lost in an extraordinarily complex, living world–a microcosm in which a beautiful but deadly ballet is conducted on a microscopic scale (Rudloe, 1971). Hungry polyps, some resembling snapdragons, others looking more like daisies or tulips, expand their knobby, translucent tentacles, slowly flexing and languidly waving them about, lulling the observer with their slow-motion ballet — until they abruptly and quite unexpectedly snap up a bit of planktonic life, stinging it, drawing it in with one violent contraction, digesting it, and then re-expanding like a blossoming flower to hunt again (Rudloe, 1971). There are many such polyps in a colony, hundreds of them, each of which is armed with many tentacles and countless nematocysts, and at any given moment, some of them will be dormant and still, some will be expanded and lazily casting about for prey (Rudloe, 1971), and still others actively feeding (Abbott, 2003)."
The feeding or nutritive zooids are the distinct individual animals in a hydroid colony that are responsible for capturing and digesting prey; as such, they bear the nematocyst-studded tentacles. But you need high magnification in order to appreciate the true beauty of living hydrozoans, or to differentiate between different species of hydroids, or to observe the zooids going about their deadly business.
Hydroids are insidious because they start out so small and insignificant, yet spread so quickly under ideal conditions (e.g., a nursery tank or dwarf seahorse tank receiving daily feedings of Artemia nauplii). Many species can spread asexually by fragmentation as a microscopic speck of the parent colony. All of the troublesome types have a mobile hydromedusae stage, which look like miniscule micro-jellyfish, and can spread sexually in this way as well (Rudloe, 1971). The mobile medusae swim about with a herky-jerky, pulsating motion and are often mistaken for tiny bubbles due to their silvery, transparent, hemispherical bodies (Rudloe, 1977). These tiny jellies often go unrecognized until they begin to settle and are discovered adhering to the tank walls. They will have a large "dot" in the middle of their bodies and smaller ones at the base of their nematocysts (Abbott, 2003). (These are the nasty little guys you have a near six-gallon nano tank, mermaid, and they are serious threat to your seahorses.) Both the polyp stage and the medusa stage sting (Rudloe, 1977) and are capable of killing or injuring seahorse fry. Multiple stings can kill the babies outright, but they are often only injured by the nematocysts, which damage their integument and leave them vulnerable to secondary infections. Many times it is a secondary bacterial or fungal infection that sets in at the site of the injury which kills the fry.
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 aquarium, there are a couple of good ways to eradicate them and get the situation under control again, mermaid:
Eliminating Hydroids — the Heat Treatment and medicating the tank with Panacur
Some types of hydroids 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 so that the hot water circulates throughout them and destroys any hydroids or hydromedusae that may be present in the filtration system. (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 is a more reliable way to eliminate them. If your particular hydroids do not resemble snowflakes, mermaid, then there is a chance that the heat treatment will be effective.
The other alternative is to treat the tank with fenbendazole (Panacur) while the seahorses and macroalgae are present, as explained below:
Eradicating Hydroids Using Fenbendazole (brand name Panacur)
Hydroids can also 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.
However, there are a couple of things you should keep in mind when treating an aquarium with fenbendazole, mermaid. 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, if no baby seahorses are present in the aquarium. 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.
If baby seahorses are present, reduce the dosage to 1/16 teaspoons of 22.2% fenbendazole granules per 10 gallons of water, and administer three doses every other day, just as described above.
Because fenbendazole is essentially a de-worming agent, it will destroy any bristleworms, flat worms, spaghetti worms or the like, in addition to hydrozoans.
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! (You will need to remove your gorgonia and any other live corals during the treatment period, mermaid, and then be very careful when you return them to the aquarium to make sure you are not giving any remaining hydroids the gorgonia could be carrying a free ticket back into your seahorse 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. But it is deadly to most type of snails. For example, Trochus or turbo snails, Astrea snails, and especially Margarita snails are sensitive to the medication and must be removed from the aquarium until the treatment regimen has been completed and the fenbendazole has been pulled from the aquarium using activated carbon and/or polyfilter pads for chemical filtration. On the other hand, Nerites, Ceriths, and Nassarius snails are less sensitive to the medication and can remain in the aquarium during and after treatment with fenbendazole. If you’re not certain what type of snails you have, it’s best to remove them until after the hydroids have been eliminated and any remaining traces of the fenbendazole/Panacur have been removed from the aquatic.
Fenbendazole (brand name Panacur) will kill starfish but crustaceans such as copepods, hermit crabs, and shrimp are normally not affected at all.
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 of the worm population.
In short, fenbendazole is an excellent way to eliminate hydroids from dwarf seahorse tanks, as well as nursery tanks and rearing tanks or anytime they take over an aquarium and cause problems. If there are hydroids present, needless to say the newborns and small seahorses will find them very irritating, and may be killed outright as a result of multiple stings, depending on what type of hydrozoans you are dealing with.
Okay, mermaid, that’s the quick rundown on hydroids (both the colonial polyps and the mobile Medusa stage or micro jellyfish that you have) and the recommended procedures for eliminating them from your aquarium. In your case, I would set up a hospital tank with freshly mixed saltwater, give your seahorses a quick freshwater dip for external parasites, and then transfer them to the hospital tank for treatment with Nitrofuracin Green while you are eradicating the hydrozoans from your nano tank.
Best of luck eliminating the hydroids and getting your aquarium up and running again in good order, mermaid!