what type of adjustments should be madeto make a lens invisible by putting it inside a water tank

This question is one of my favorite questions

The answer to this question is astonishingly simple.

See do we see air in air? What??????????

The same thing can be used in this case

if the lens's material would just pass undeviated .

Even better take the lens formula(not the simple one, but the one that involves u of medium and lens)

equate both u(lens and medium). see that the image and the object coincide.

Hope you understand.