If the light coming from bottom of a water tank does not come out of the water surface, it implies that the ray of light has undergone total internal reflection. When a ray of light travelling from a denser medium to a rarer medium is incident at an angle greater than the critical angle for the pair of media in contact, the ray gets totally reflected back into denser medium.

So for the light ray from the bottom of tank to not come out of the water tank, the angle of incidence should be greater than the critical angle for that pair of media. 

The minumum angle of incidence called as the critical angle is related to the refractive index as:

So the minimum angle of incidence should be 48.59°