There is an absolutely effective remedy against Hydra and Planaria. The entry in Wipikedia says: “Flubendazole is an anthelmintic. Its brand name is Flutelmium which is a paste manufactured by Janssen Pharmaceutica N.V. used by veterinarians for protection against internal parasites and worms in dogs and cats. Other brand names are Flubenol, Biovermin, and Flumoxal.”
Many of us in Europe have used it (here it is called “Flubenol”) as a white powder with definitely sure success against Hydra even when applied in very small doses. In a tank of about 25 liters you only have to put a very small amount (“a knife’s point”) on the water surface, it will spread there and one day later you will see the Hydra contract themselves and disappear. I never had fish loss or fish indisposition after its use.
But some aquarists dissuade from using it in presence of shrimps, others do not assent. I should be careful with that.
You should provide for a small amount. Try to get the powder-form, it’s easier to apply.
I do not know whether there is an aquarium-brand of this product in the U.S. Here in Europe there is none.