Hydra's,....well I have experience with them
Hydra's are named after a myth abouth a monster with 10 heads. The problem with hydra's is that they have 3 way's to multiply. By seed in the water, splitting up cells and a baby is born on the foot of the hydra and next is that parts of the hydra can grow out to complete new hydra's. So when you scrape of the hydra's and they are broken into 3 parts, those parts probably will grow out to 3 new hydra's. Well,....probably your not to happy to hear this but don't worry!!! You can control the growth of the hyda's a bit by not overfeeding! An other reason can be an explosion of microorganismes in the water witch is quit normal in a new set up. When the tank is stable the micro organisms will reduce in number and become stable. And the only thing that kills hydra's is a med named Flubendazole 5%. This med is used for worm treatment and available with the vet and in some country's also in the lfs. Flubendazole melts the Hydra's but it can also kill snails and elephant fish. Flubendazole is a mild med that is quit save to use. The only problem is you got some small fry! I don't know how many hydra's you got and what type. The most common are the small green once of abouth half an inch. They don't be a treat to your fry. The larger types (white, gray) can grow over an inch and do feed on fry as well. If you have the small (green) type I suggest to wait a couple of weeks so the fry are a bit larger and can handle more. If you got a real explosion you could treat them with the Flubendazole but keep in mind that there is a chance this can effect the fry.
Flubendazole is available in a solution (liquid) but this type can't be stored for a long time and will lose strength. The powder would be the best choice. Before you put in the meds you need to do a large water change of abouth 40%. You can take a cup of tank water and add the powder. A dose of 100mg / liter = 26 mg / gallon is quit save to use and effective on killing the hydra's. If you have water with a KH of 2 or lower you need to add the meds over a longer time span (several hours). Mix the powder in the cup well. That's not easy with Flubendazole becouse it doesnt solute that well in water. When it is mixed you can spread it over the tank water surface. You need to create enough water movement on the surface and if possible add an air stone. After 48 Hours I suggest to add 25 mg / liter = 7 mg / gallon to the tank. after 5 day's all the hydra's should be melted and reduced to a dot on the glass or completely gone. You can do a large water change again and preferable add some liquid bacteria to the tank. The flubendazole will kill some of the bacteria in the filter and also kills some of the micro organisms (worms and shrimp related creatures) as well as snails. This can cause a mini cycle becouse of increased waist levels and reduced bacteria. I have never had problems with adding the flubendazole whether it was for worm treatment or killing hydra's. I also suggest to hear other opinions on the illness board.
