I'm afraid your tank is far too small.
I don't know what a 'demon queen' fish is but, as Muroco points out, paroon sharks can grow to 6 or 8 feet long. You would need a swimming pool or massive pond to keep them.
The best thing you can do is take them back to where you got them, or they will die, I'm sad to say.
Can you post a picture of the 'demon queen' fish, or do you know their scientific name? They are lots of fish that can live in an 8 gallon tank, but they need to be SMALL!
If you have a small tank, you must get small fish! If you want big fish, you must have a big tank!