It looks like Myriophyllum aquiticum, commonly known as Water Milfoil. It is highly invasive in natural waters in Europe and North America, and I believe many jurisdictions prohibit it. Just make sure none of it ever gets down the drain and/or into the ecosystem. Destroy dead leaves in the compost or garbage,not down a drain or toilet.
It looks like Hornwort to me, great plant but can also become invasive, prefers cooler waters but can grow in a tropical tank either planted or floating I keep some in an outdoor pond over winter, it survives well by becoming dormant so it dies down but then regrows from tiny buds in the soil once the water starts to warm up again in spring.