Carbon Filters

ive got a 12 inch carbon filter with a 4 inch inlet/outlet. my inline fan and my lights are all 6 inch. is there a way to convert my 4 inch to 6 inch or can i just put a reducer on it.

I think you answered your own question there. Get a reducer.

Every carbon filter I've used is specific for a certain CFM. I believe you could just change the flange itself on the filter, I know I could on mine.