There are several ways to get rid of hydra... certain types of fish eat them, you can electrocute them or you can add certain medications to eradicate them (I used panacur). Do a google search and many options will open up for you.
Most fish places have "Fluke Tabs". Use 1/2 tablet for each 10 gallon water. I usually crush them and mix it in small glass of aquarium water and just dump it in. Hydra will be gone very quickly. Won't bother fish or plants.