i have Ehiem 3 canister filter in my 120gal. I have never had a canister filter before and my tank is starting to smell a little and im not sure how to fix the issue. there is a green tint to it that I cant seem to get rid of with weekly water changes ect. im sure there are a lot of different things but what the best medium to be using. I have two small Oscars,2 small jds, 1 Green Terror, 1 green texas and a jag.