I recently had a hydra outbreak in my crs tank and I was trying to find a way to eliminate them. There has been many mentions of Flubendazole and Fenbendazole being used successfully as treatments but unfortunately these drugs aren't commonly available in Canada without a vet prescription. However one medication is readily available, Mebendazole, which I found out after some research is one of the two drugs in Fluke-Tabs.
My main concern was sensitivity of the shrimp and snails in the tank so I followed a similar very low dosage as is done with Fenbendazole to great success!
The tablets I got were 100mg Mebendazole per 310mg tablet, so 1mg of Mebendazole per 3.1mg of tablet. I've read that people were successful with a dosage of about 22.2mg per 10g of Fenbendazole in shrimp tanks so I decided to dose approx 20mg of Mebendazole in my Fluval Ebi which is approx 8 gallons. To dose 20mg of Mebendazole I needed 62mg of a tablet, so I just kept cutting a little bit at a time off the tablet until I had 0.06 g on my scale. After that I just pulverized the chunks of tablet in a small plastic cup and filled it with tank water and tossed it in the tank. Mebendazole doesn't dissolve very well just like Fenbendazole but after 24 hrs the white specks were almost all gone.
There was no immediate reaction from the hydra, nor any of the shrimp or snails in the tank, but after 24 hours the hydra were all shriveled up obviously being effected by the drug. Again no reaction from the shrimps or snails, they were all active and happily moving around looking for food like normal.
After 48 hours I dosed a second dose of 20mg just to make sure there are no hydra survivors that may start to develop some resistance and again absolutely no ill-effects on the snails or shrimp (babies and adults) but no signs of hydra survivors either.
So Mebendazole seems to be shrimp safe, like Fenbendazole as long as it is dosed in low quantities. This has just been my experience with lower grade crs (C to A grade) so take extra precaution if you have more sensitive shrimps and watch them carefully for any signs of stress if you choose to try Mebendazole.