If your eel swims up your pipe that water flows out of into the tank...
Here is a cheap simple trick
You'll need:
* 1stocking
*small elastic bands
*scissors
Instructions:
Cut a small piece of stocking that goes over the pipe to cover on the outside and hold it in place with an elastic band. It allows water to flow out and keep eels out.
You may need to change it when needed.
It is a good temporary solution.
No harm should be caused from using a fabric, my eels have been doing well for months with that over the pipe.