Project

How about a load cell on each corner of the tank, and weigh the contents.
Hx711 load cell amplifier modules can be got from ebay and are compatible with arduino, you can also get load cells on ebay.
One consideration if the tank is outside is wind load, you'd need to make many measurements and work out an average to counteract forces from the wind.

Let's say a leakage of 5 liters/day from a tank of 10.000 liters capacity, could be not detected at all. But, if the sensor is sensitive enough, vibration coming from any mechanism in the vicinity could affect level measurement.

In a tanker vessel, I had the chance to be inside of a cargo tank, half full of ballast. It was easy to perceive the effect on the surface, from someone hammering something on main deck maybe some 10 or 20 meters away from the access.

In tankers, ultrasound (is sometimes used). There is also the technique to detect the water-fuel interface.
The most common I have seen, for rough measurements in tanks ashore, are of the float type.

Google "radar".

Or more concisely 'guided wave radar'
Not a subject for a beginner.

Mmm ok

Probably better with ultrasonics..

eg. Have a look at something like this; range up to 4m and easy to use with any microcontroller:
https://thepihut.com/products/ultrasonic-distance-sensor-hcsr04

Just send a trigger pulse and time the delay until it picks up the echo.