Hello Allwet and welcome to the forum!
I had to research your question - appears these are an eel-like, omniverous amphibian and fish are indeed a preferred forage item. I don't have experience with these organisms, but I would not be overly concerned regarding their impact on your fishery. Many predators of fish exist, in my experience unless it's an otter, heron, pelican or cormorant, one doesn't have to be too concerned with their impact on the fishery. Your tilapia should be fairly adept at avoiding predators, and they'll probably quickly grow beyond the gape of the Siren. Your tadpole population is likely so dense you would never notice an impact on their numbers, either. Personally, I'd not lose much sleep over it, but keep an eye on them and try to provide some video of them hunting - the forum would love to see them in action. We welcome any new science to add to our archives!
When time allows, tell us more about your pond and your goals and we'll do all we can to help you achieve them.
Many men go fishing all of their lives without knowing that it is not fish they are after. ~ Henry David Thoreau