The Hydrophilidae family currently has over 2,800 known species, divided into two main types: aquatic and terrestrial. Aquatic water beetle larvae are mostly predatory, feeding on various small invertebrates in the water (shrimp are invertebrates); the diet of adult aquatic water beetles varies by species, and some water beetles are good swimmers.
Water beetles are common aquatic insects in paddy fields. If there are fields near your home, you have a higher chance of encountering water beetles. If you have aquatic plants in your tank, be careful with quarantine, as some aquatic plant farms grow plants outdoors, so it's safer to quarantine them before adding them to your tank. I first discovered water beetles in my aquarium under a water hyacinth leaf. At that time, to give the water hyacinth enough light, I had placed it outdoors in the sun for about a month, and there happened to be rice paddies nearby. This might be why they ended up in my tank.
Adult water beetles can be as large as scarab beetles or as small as a peanut. It's difficult to determine what kind of adult a larva will become by looking at the larva. To the naked eye, their larvae look very similar to caterpillars, so to be safe, it's better to remove them if you see them.
The lesser water scavenger beetle (Sternolophus rufipes) has carnivorous larvae that prey on mosquito larvae, while adults are omnivorous. They store air on their ventral side. They are about 9 mm to 11 mm in length and can be found in calm low-altitude waters throughout Taiwan.
Some water beetles have very high salt tolerance, such as Berosus tayouanus Ueng, which is about 4 mm to 6 mm in length. The larvae prey on aquatic invertebrates, while adults feed on decaying matter in the water. It is one of the most salt-tolerant insects currently known in Taiwan and can be found in coastal tidal channels, abandoned fish ponds, salt fields, and other water bodies.
References
Liu, Hsing-Che (2018), Aquatic Insects of Taiwan Atlas. Retrieved from: https://dytiscidaetaiwan.blogspot.com/search/label/%E6%B0%B4%E7%94%9F%E6%98%86%E8%9F%B2%E5%9C%96%E9%91%91 (accessed 2021/10/26)
Liu, Hsing-Che (2018), Sternolophus rufipes (Fabricius, 1792) Lesser Water Scavenger Beetle. Retrieved from: https://dytiscidaetaiwan.blogspot.com/2017/06/sternolophus-rufipes-fabricius-1792.html (accessed 2023/01/16)
Minoshima, Yusuke; Hayashi, Masakazu (2011) Larval morphology of the Japanese species of the tribes Acidocerini, Hydrobiusini and Hydrophilini (Coleoptera: Hydrophilidae), Acta Entomologica Musei National Pragae, 51 (supplementum), 10.
Cheng, Ming-Lun (2014), Taiwan Encyclopedia of Life. Retrieved from: https://taieol.tw/pages/4798/articles (accessed 2023/01/16)
Taijiang National Park, Berosus tayouanus Ueng, Wang & Wang. Retrieved from: https://www.tjnp.gov.tw/Encyclopedias_Content.aspx?n=557&s=251289 (accessed 2023/01/16)