Worms and Hydra
I set up my new 125 liter aquarium (80 cm x 35 cm x 50 cm) with plants and decoration 6 weeks ago. Last week, I suddenly discovered 2 different, strange white animals. One species is about 5 mm large, white, with an elongated shape, looks like little threads and creeps around on the aquarium glass. The second species has the same size, "flies" through the aquarium and has 4 "arms" at one end. I already got some information: Is it correct that they are flatworms and Hydra?
flatworms are one possibility considering the animals that creep on the glass pane, however threadworms (nematodes) or oligochaetes are also possible.
The shape you described for the animals that live freely in the water matches Hydra, the description of the behavior, however, does not match. Hydra sits on the panes or on another substrate and does not swim around freely - all I could imagine is that they are not yet attached, still floating specimens or ones that were torn off.
These aquarium inhabitants are usually not harmful, except that they might prey on small fish fry (Hydra) or eggs and not yet actively swimming fish larvae (flatworms). The above mentioned nematodes (according to the way they appear they are definitely not parasitic species) and oligochaetes are entirely harmless.
I read everywhere that you can introduce this with live food.
This is one possible source; however, you can also get them into the aquarium as eggs or larvae along with plants. Some of these invertebrate animals are so resistant that even severed body segments may regenerate to form entire animals again!
I want to have the problem under control in any case before I add fish!
It is really not a problem; these or other invertebrates are present in more or less large numbers in virtually every aquarium (just put a small sample of filter sludge under a microscope: it contains much more than "only" bacteria!). Combat, if it is actually desired, is often also possible biologically by simply not feeding the fish for one or two weeks.
According to my retailer, the water parameters are generally alright: pH 8; 10 - 16 °dGH; 15 - 20 °dKH; NO2 not detectible; NO3 0 - 10 mg/l.
They are good for livebearers and fish from Lake Tanganyika; however, softwater species should not be added.
Do the snails and the other animals multiply so rapidly because there are no fish in the tank yet? What to they live on then?
These animals basically feed on decomposed plant parts that always form in the beginning. Hydra mainly preys on small animals, e.g. tiny crustaceans.
Dr. Bodo Schnell