Green or brown film grows inside of tubing in canister filters. Here is a way to clean them and leave them as new. You will need bleach, 1 bucket, and a spare power filter.
(1) Take out the media from your filter. Clean the media and place part of it in the power filter and the rest in a corner of your aquarium.
(2) Place a bucket with clean water high on a piece of furniture (table, chest of drawers, etc) and place the inlet and outlet tubings of your filter in the bucket.
(3) Purge the filter and turn it on. Let the clean water flow through your filter. Add more water to the bucket if necessary.
(4) Add one cup of bleach. If doing this indoors you must cover the bucket with plastic wrap. I use Glad Press'n Seal to seal it perfectly and avoid chlorine odor in my apartment.
(5) Let the filter run for 48 hours.
(6) Discard the water and refill with clean water and add plenty dechlorinator. Run the filter for 6 hours and discard the water.
(7) Put media back into the filter and install the filter in your tank again.
I hope you find this helpful. It has worked great for me.