Hydra are one of my favourite organisms of all time. They are very noticeable and have a very unique appearance. They will often be found anchored to the glass in your ecosphere hunting for copepods, which is their main food source. The population of these organisms will fluctuate as copepods become rarer or more common, due to their heavy reliance on them. They will never be able to make the copepods extinct due to the copepod’s ability to go to places where the hydra can not. The hydra and copepod population should develop a cycle over time. In my opinion, this is one of the most fascinating processes of an ecosphere, as I have seen the copepods come back from near extinction and vice versa. Hydra are harder to find than other species. They can often be found clinging to duckweed or other native plants in ponds and some slow moving rivers. There are 2 main types you will come across: Brown hydra and green hydra. I recommend only sticking to brown hydra because green hydra can survive on algae (hence their colour) which could cause the copepods to go completely extinct due to the fact that their population will not go down as the copepod population does. I really recommend trying to get this species in your ecosphere. However, I would try to add this only once a stable copepod population has been achieved.