6-7 years ago, i used to handle lots of formalin based chemicals when treating my saltwater fish for parasitic infections. will this increase my risk for bladder cancer? i dont handle it anymore but there were times where i had skin contact with it?

Please review the following like for chemical exposure and workplace risks for cancer. If you identify concern, you and your provider should discuss your history and risk factors and considerations for cancer surveillance: https://www.atsdr.cdc.gov/emes/public/docs/Chemicals,%20Cancer,%20and%20You%20FS.pdf

To cause cancer a chemical has to be ingested, absorbed or come directly in contact with the bladder for years (many). So unless you ingested it or absorbed it for a long time than your chances of getting bladder cancer from formalin are extremely low to nil.