A water level sensor without the need to put a hole in your water tank. The hall sensor or reed contact is placed outside of your tank!

Place some magnets into the hole and glue it or print the a lid and glue the lid onto the float part.

For example I use the sensor on the bottom of my water tank to check if water is available to run the pump to water my flowers.

After you have placed or glued the sensor into your water tank, use a hall efect sensor or a simple reed contact (outside of the tank!) to check if the magnets are available.