Common Name: Hydroid
Scientific Name: Hydrozoa
Hydroids are a life stage for most animals of the class Hydrozoa, small predators related to jellyfish. Most hydrozoa species include both a polypoid and a medusoid stage in their lifecycles, although a number of them have only one or the other. In the polypoid stage they can be seen attached to a rock most likely in a group of many individuals. In the medusoid stage hydroids look similar to very small jellyfish type creatures. The majority of hydroids in saltwater are colonial and often have both stages in their lifecycle. Hydroids are often discovered in the tiny jellyfish-like stage while swiming around in low flow areas in the tank. Low flow aquariums , such as seahorse tanks, fry tanks and larval rearing vessels often get a hydroid infestation with an addition of a new live rock or other items to the tank. The bad news is, they can sting and kill dwarf seahorses and fish fry that get too close to them. Hydroids love to eat baby brine shrimp and can be introduced to the tank on almost anything, including the very ironic way of the shells of brine shrimp eggs. In the aquarium hobby there are two common Hydroid types: one will be in a form of small white creatures and the other of bigger individuals with more brown colorations. if you discovered the small white ones in you aquarium (depicted in the pictures above) then you have the less invasive and less dangerous kind. If you are unfortunate enough to have the other kind(more brown in coloration), please keep reading for some more information.
Common Opinion: BAD (but depends on your situation)
It is important to note that many aquariums, “new” aging saltwater systems at around the 8 month mark, get a bloom of white small hydroids that eventually disappear with time. These are a problem if you are raising some fish from eggs, clowns and bangaii Cardinals don’t seem to mind (Tested by AquariumJunkiesCanada). Seahorses have a problem with all Hydroid types as they sting the young. If this is not your case or you still want to try and get rid of your Hydroid, try the following:
Make sure you are using decapsulated brine shrimp eggs, or decapsulate them yourself. You can buy already decapsulated brine shrimp eggs from most fish stores. hydroids don’t respond to common parasite treatments like low salinity but have a drug that is accepted as a proven treatment. The proven drug that helps in treating hydroids is called Fenbendazole (Panacur). However, you should familiarize yourself thoroughly with this drug before using, or better yet, DO NOT use it at all. Wrong dosing amounts of this drug can kill all your aquarium inhabitants. in addition, most invertebrates and corals DO NOT tolerate Fenbendazole and will die. If the hydroids are on a rock that can be removed, take the rock out boil it and let it dry up for a couple of months.