Re:Ringed Top Snail? Eats Hydroids?

Pete Giwojna

Dear Carrie:

I’m sorry to hear that the dreaded ‘droids have colonized your nursery tank and dwarf seahorse setup, but I do think there are a couple of ways you can deal with the situation short of breaking down the tank and sterilizing everything and starting over in a new setup.

I’m not sure what the diet of the purple ringed top snail might be, or whether they would be inclined to graze on hydroids, but that’s probably a moot point because I have never seen them offered for sale anywhere either.

If you cannot obtain purple ringed top snails and you want to avoid treatment with fenbendazole (Panacur), 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 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,

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. Some folks might describe the snowflake type of hydroids as "fuzzy starfish," in which case the heat treatment may not be effective. If you’re fuzzy hydroids do not resemble snowflakes, then there is an excellent 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. Unless you are using live rock in your Pixie tank, Carrie, your snails should be just fine. 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. So as long as your snails are not a species of Astrea, they should tolerate the treatment with fenbendazole without a problem.

Likewise, by all accounts, 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.

However, hobbyists certainly do need to 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)!

So I think it would probably be quite safe to treat your Pixie tank with fenbendazole while the snails and macroalgae are in it, Carrie. Nassarius snails are not affected by the Panacur at the usual dosages and are the most common types of snails to be used as sanitation engineers in a dwarf seahorse setup, and most types of macroalgae don’t mind the medication at all either. In order to be sure of eradicating the hydroids, it’s a good idea to treat the snails and the macroalgae, since they may well be harboring either the colonial or hydromedusae stage of the hydrozoans.

If you decide you want to try the Panacur, Carrie, the recommended dose for fry tanks and Pixie tanks with juveniles is 1/16 teaspoon of the horse dewormer granules (22.2% fenbendazole) per 10 gallons of water. Dose the tank with 1/16 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. You will see the hydroid colonies began to droop and wilt as the medication takes effect.

Best of luck eliminating your hydroid problem, Carrie!

Pete Giwojna

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