The biggest thing you need to worry about is if you can't determine what they look like, look for very thin tentricles comming off of them. The small form of a Hydra looks like an anthipod (or whatever) and will grow rapdily to destroying your tank. NOt to ruin your belief but becareful. Although Hydra are most common in freshwater, and are ill to salt (or they say)... ironically, there have been numerous reports of this multi headed beast in salt water aquariums. At night i recommend you turn off your light, grab a flashlight and a magnifying glass and look at them if you can find them. Look for anything moving like hairs. If they don't have any of these hairs you should be fine... for if you look at them in a magnifying glass you will know if its an anthripod or a hydra; they look completely different upclose. If it is a hydra; then there are numerous things you can do. But what i recommend is the slowest acting beasts on the planet; BRING IN THE SNAILS. ALthough the possibility of snails killing them totally off is slim, they will leave you alone because snails will do them in.
***THe creature i am talking about is not really a Hydra, its a name used for a fresh water creature that has carried on to a new saltwater beast.