You should read this link:
http://en.wikipedia.org/wiki/Hydra_%28genus%29
The ones in Biotope are probably gone by now. First time I saw them, was when the store was back at Bukit Merah. You can always ask around for people who have them, to "give" you some. They always appear frequently, if you feed too much crustaceans to the fish. Sometimes they are just hitchhikers on aquatic plants.
Perhaps a visit to a nearby stream, or pond near your home, where you can see water plants, might net you some specimens. I would try the Science Centre ponds.. used to catch all sorts of weird animals from the water.
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