[Pavel Chaloupka]

This problem is hard to avoid from time to time if you use natural plancton. Some fish are able to eat them if they have nothing else to ffed on. Did you try that? If you have no experience with using some kind chemicals with Paros I would use the good old way that is very efective and safe. If you have any kind of hydra that does not have symbiotic algae to help them survive (is not green) you can try to starve them to death first. . Just use for example black (Culex mostly), white (Chaoborus) or other kind of mosquito larvae. Something big that they will most likely not catch becouse it will not spread over the tank before its consumed by fish. It sometimes works well if your fish are eager enough eaters, unless you have alot of infusoria or rotifera living in your tanks. Limitation of light would be necessary with the green ones. In such case remove the fish as no light for several days would not be good for them , natural fotoperiod is very important. If you are unsuccesful starving them this way, use a little piece of soft copper (some kind of wire or so) and just hang it in the water of the tank. Hydras tentacles will start to shorten and as they loose them you can already remove the copper. Hydra will die of starvation.In the very soft and acidic water this could take hours and the copper disolved in the tank water will bond to organic matter like detritus, peat, leaves and stuff. It works well enough to kill the hydra before the level of disolved copper starts to be dangerous for fish. If you want to be realy safe, just remove the fish for the treatment. How quickly it works is dependent on ammount of organic material in the tank for the copper ions to bond. If you clear the tank it will work faster but of course the level of toxic copper ions will be higher.Do not use it with eggs in the tank. This way you dont have to risk contamination of the tank environment with potentialy harmful substances. I hope that helps. + If you want to prevent the hydra from getting in to your tanks, next time you bring the plancton home, leave it in some bucket for like an hour and than pour it somewhere else. The hydra will attach to the first bucket and you dont have to solve this problem that often.