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Re: Hydra like things...
>They are light yellow/green in color. They seem to grow on a shared
>stalk like some marine polyps, and one little guy is at each node.
>Size is about <1/4" tall. The shared stalk is about the size of a
>human hair (though a little smaller). I can definitely see their
>little arms when they are in the water. I've yanked them out, but
>saved them (as I'm not quite sure if I want them or not).
Hydras are solitary, not colonial, as you already know, but there are
freshwater colonial polyps. The genus found in North America and
Europe is Cordylophora. I suppose it could also be a bryozoan, as
some can resemble polyps. Bryozoans are far more common than
freshwater polyps. A good reference on this topic is "Freshwater
Invertebrates of the United States", by Robert W. Pennak.
They shouldn't harm your fish or plants, but bryozoan colonies can get
pretty large (some can cover several square feet). I suppose a
heavily planted tank can produce enough detritus, algae, protozoans,
etc. to support a large bryozoan colony. Freshwater polyps rarely
exceed 100 mm in length.
Nothing to worry about, but plenty to marvel at.