Aquarium kinda thingy

How can i put water inside cube or some other object(basically an aquarium) and then put some other object in it which have less gravity.

there is no such thing as “water” in 3D graphics, you’ll have to play around with transparent shaders and see which one fits the look you’re trying to get best. My first guess would be to use a transparent/diffuse shader with very low alpha value.

As for the gravity, give the aquarium a trigger and apply an upward force inside the OnTriggerStay function, something like this:

void OnTriggerEnter(Collider c)
    if (c.gameObject.rigidbody)
        c.gameObject.rigidbody.AddForce(Physics.gravity * -1.2f);