water smells

Question
QUESTION: I have a 210gal freshwater tank and have been working with freshwater fish now for about 6 years now.I have several large fish in my tank, but it is not overcrowded. I do water changes on a regular basis. 25% changes about 2 times a week. I still cannot get rid of the fowl odor. My fish seem to be healthy and eat regularly. What do you suggest that I do? Please help me.

ANSWER: Mark,\
I would need to know what you are running for filters, what kind of fish, what you feed.
Under normal conditions a 20% water change once a week would be plenty.
Lets see what we are working with here and perhaps we can come up with a solution.

---------- FOLLOW-UP ----------

QUESTION: I am running 2 RENA XP3 and a large bio-wheel. I feed frozen blood worms, TopFin flake food, dried shrimp,ciclid floating pellets.  1 Turquoise Faced Severum, 1 Do-Jo, 3 Parrot Cichlids, 1 Chinese Algae Eater, 2 Rainbow Sharks, 1 Chocolate Plecostomus, 1 Sunshine Severum, 1 Jack Dempsey, 1 Grey Texas Cichlid, 1 Black Shark,  1 Albino Cichlid, 1 African Cichlid, 1 Lion Head Cichlid, 2 Black Marbled Angelfish, 2 Silver Dollar Fish, 3 Plecostomus, 1 Australian Rainbow,  1 Giant Rainbow Neon, 1 Corydoras Catfish, 5 Guppies.


Answer
Mark,
have you done a gravel cleaning?
also, in the cannisters are you running any carbon?
If you can get some decent carbon loading the cannisters with it will take out the smell.
However that being said, we should find out what is causing the trouble and eliminate that.
Not a ton of fish in the tank considering the size.
I am thinking that perhaps the Rena;s are abit outgunned on this tank.
I would change the cannisters and try that, if that does not work or make a difference you could add a small UV in line around a 10-15 watt that will kill free floating bacteria as well.
Remember the rena;s flow rate diminishes quickly due to clogging, and height of lift.
At some point you may consider a more efficient filter system, I would look at a Ocean Clear cannister with a 18Watt UV built in.
You could keep the renas as well as back up and the OC could handle a tank 3 times plus that size.
If you need further info, just ask.