There's not much you can do about a crack in a fiberglass bathtub. These cracks can be caused from people jumping into the bathtub, while its empty or full, but on the other hand, I'm sure there's hundreds of possibilities.

I watched a fellow worker dropped a 2 x 4 about 8 feet and it actually put a hole in the fiberglass bathtub. This bathtub was also repaired and should have been replaced, however, I did return to this home, about three years after we were working on it and the bathtub repair still looked good.

This bathtub has already been repaired once and as you can see it in his cracked again. The problem itself isn't the bathroom crack, it's the damage it did to the ceiling and is continuing to do, until the proper repairs have been made.

In a situation like this, I would recommend replacing the bathtub.

Home Repair Tip -I rarely see a plumber or remodeling experts install bathtubs with plaster of paris or cement underneath them. This provides additional support for the weight of the bathtub when full of water and could have prevented the damage to this fiberglass bathtub.