If its getting bigger that rapidly it is likely a Non-Neoplastic Tumor...i.e. not an abnormal proliferation of tissues as such not Cancer (yes, fish can get cancer), cancer (weather malignant or benign) don't grow that fast....There are a few things that can cause a non-neoplastic tumor....Inflamation (hopefully thats it from bumping into something), edema, malformation, a cyst, or a hemorage....To my knowledge, nothing you can do other than wait and hope. But like rebel said, Sep and Beth are the knowledgeable ones on that.