I would never recommend any water beetles in your ecosphere unless you know what you are doing. Many of them are heavily predatory and therefore are harmful to your ecosphere. However, species within the Hydrophilidae family, commonly called water scavenger beetles have shown promise within ecospheres. I discovered a colony of berosus spinosus within my own ecosphere and they seem to be doing well. Water scavenger beetles, unlike their predatory cousins feed off dying plant matter and detritus. They are able to spend long periods of time underwater by carrying a bubble of air on their underside. The beetles I found were very small, about 5 mm (aprox 0.19685 Inch)  in length. Most water scavenger beetles can be characterised with clubbed antennae. As mentioned earlier, I do not recommend trying to find these beetles unless you know what you are doing. However, if you do find some, they are one of the coolest organisms that can be put in an ecosphere.