WaterProof Sensor School Project (in a small water tank)

Hi,
I'm working on a school project and I intend to build a system to measure distance in a small tank of water and therefore to be able to detect objects placed in the tank (such as a box).
I just need a sensor that works underwater. I intend to leave at first the water very still and clean at first and ultimately change different factors to see if the results are different.

I looked up on the internet for different sensors but they all seem to only work for air use.
If you have any information, advice, article or tutorial about how to build this system, could you please send me the links.

Thanks,
Alex

Google is still your friend here.
If you talking about ultrasonic transmitters /detectors , then you have to buy for liquid use - it’s not only about water proofing , but the interface to get the sound in the water difffers from air types. .
Could you not use something outside of the ( glass?) tank , eg a light source shinning across the tank.
Or maybe a coil run off an oscillator - impedance changes when metal object in the water.
Weighing the tank won’t work - see Archimedes/bath water!

How small is small?
What distance?
What precision?
If the water is clear, optical methods may work. Otherwise, sonar is your only option, not trivial to do. In a small tank you'll have to deal with lots of spurious echoes for starters. Underwater measurements are overall much harder than doing the same in air.

hammy:
Weighing the tank won’t work - see Archimedes/bath water!

Only if the displaced water is somehow removed - like flowing over the edge. The weight of the object is still added to the total weight of the tank! Not that I have the feeling it helps the OP as they appear to want to detect where the object is, rather than its mere presence.