Can fish get cancer? ?

Maybe. Moreover, the earliest paleontological evidence of cancer is fish fossils (Dinichthys, 350 million years old, and Phanerosteon, 300 million years old, Upper Devon and Upper Carboniferous, respectively). In general, finding bones with a tumor is a great success, because bone cancers are one of the rarest cancers, but, nevertheless, sometimes it happens.

I will subscribe to the question to read an expert in the future, but for now I will say one thing - even plants have cancer. Cancer is the work of mutated cells, "gone crazy", which do not communicate with the surrounding cells, do not follow the system (spit on the DNA matrix a hundredfold) and begin to divide endlessly, as if in a panic. Out of control, harming adjacent cells and infecting them with a thirst for rebellion. If you have ever seen trees, the trunks of which have undefined tumors, then this, among other things, is also possibly cancer. As for fish, I don't think there is any exception for them, since all organisms are subject to mutations. Nevertheless, I cannot answer the question with 100% certainty, since I do not have sufficient information. But, I think, yes, cancer also occurs among fish.