Many cancers are linked to obesity - breast, bowel, and kidney cancers for example. It's not entirely clear whether obesity makes developing prostate cancer more likely. But recent evidence from France suggests that once you have prostate cancer it may behave more aggressively if you are obese. It might be that fat cells around the prostate release factors that encourage prostate cancer cells to spread beyond the prostate.
As always though, the situation here is not black and white. Many are aware that omega-3 fats (found in oily fish) are linked to all sorts of health benefits. A couple of studies though have suggested that men who have high levels of omega-3 fats in their blood might actually be more likely to develop prostate cancer. There might be an association here, but an association is not the same as cause and effect. There might also be a difference between men who have high levels of omega-3 from supplements (as opposed to men who are eating fish without additional supplement pills).
So for now, being slim and eating a healthy balanced diet that includes fish, but possibly without additional supplements, is the best we can do to reduce prostate cancer risk.